Fix headings in networking.md

Update heading sizes for proper rendering.

Author: Amanda Butler

docs/reference/technology/connectivity/networking.md

@@ -1,20 +1,20 @@
-### IP networking
+## IP networking
 
 IP Networking in Mbed OS is layered in three clearly defined API levels. The diagram below shows the layers next to the closest matching [OSI model](https://en.wikipedia.org/wiki/OSI_model) layers.
 
 <span class="images">![](https://s3-us-west-2.amazonaws.com/mbed-os-docs-images/ip-networking.png)<span>IP networking</span></span>
 
 The following sections introduce the APIs and technologies implemented in each level.
 
-#### Socket API
+### Socket API
 
 The Socket API is the common API among all IP connectivity methods. All network stacks in Mbed OS provide the same Socket API, making applications portable among different connectivity methods or even stacks.
 
-In the OSI model, the Socket API relates to layer 4, the Transport layer. In Mbed OS, the Socket API supports both [TCP](https://en.wikipedia.org/wiki/Transmission_Control_Protocol) and [UDP](https://en.wikipedia.org/wiki/User_Datagram_Protocol) protocols.
+In the OSI model, the Socket API relates to layer 4, the Transport layer. In Mbed OS, the Socket API supports both TCP and UDP protocols.
 
 Refer to [Socket API](network-socket.html) reference for usage instructions.
 
-#### IP stacks
+### IP stacks
 
 Mbed OS has three options to select for the IP stack. The connectivity modules provides two built-in IP stacks or an external IP stack.
 
@@ -32,7 +32,7 @@ The following table summarizes different stacks, use cases and their limitations
 |Nanostack|IPv6, 6LoWPAN, Thread|Mesh networking, Border Router|Only IPv6|
 |External IP module|Depends on the module|(Save RAM/Flash)|Depends on the module. Usually poor match to Socket API|
 
-##### Configuring the IP stack interface
+#### Configuring the IP stack interface
 
 Depending on the Layer 3, Network layer, protocol used, there are different ways to configure the interface. It also depends on the stack used, which configurations are supported on each link layer.
 
@@ -44,7 +44,7 @@ Depending on the Layer 3, Network layer, protocol used, there are different ways
 |Nanostack|Ethernet|IPv6|static or [RFC 4862](https://tools.ietf.org/html/rfc4862) IPv6 Stateless Address Autoconfiguration. No DHCPv6 support|
 |Nanostack|IEEE 802.15.4|6LoWPAN|Thread or 6LoWPAN-ND+RPL|
 
-#### Network interfaces
+### Network interfaces
 
 Network intefaces are the application level APIs where users choose the driver, connectivity method and IP stack. Each connectivity methods requires different configuration,so these APIs are not interchangeable. The application developer must choose one. Choosing the interface also automatically pulls in the network stack as a dependency.
 
@@ -60,29 +60,29 @@ Mbed OS implements the following network interface APIs:
 
 Refer to [Network Interface](network-interfaces.html) API reference for usage instructions.
 
-#### Network driver
+### Network driver
 
 The network driver is a generic term to describe different APIs for connecting networking device to the IP stack or Socket API. Below is a description of each driver API.
 
-##### Ethernet driver
+#### Ethernet driver
 
 <span class="images">![](https://s3-us-west-2.amazonaws.com/mbed-os-docs-images/emac.png)<span>Emac API</span></span>
 
 Ethernet drivers are implemented using the stack-independent EMAC API. Because the Ethernet driver requires no configuration, it does not implement any controlling interface for the application.
 
-##### Wi-Fi driver
+#### Wi-Fi driver
 
 <span class="images">![](https://s3-us-west-2.amazonaws.com/mbed-os-docs-images/wifi.png)<span>Wi-Fi driver</span></span>
 
 There are two types of Wi-Fi drivers in Mbed OS, depending on which protocol layer it implements. Wi-Fi drivers are either a special case of Ethernet driver or external IP stacks. Wi-Fi drivers require configuration from an application and, therefore, implement both the low level EMAC API or Network stack API and the high level controlling interface API called `WiFiInterface`.
 
-##### Cellular modem driver
+#### Cellular modem driver
 
 <span class="images">![](https://s3-us-west-2.amazonaws.com/mbed-os-docs-images/cellular.png)<span>Cellular driver</span></span>
 
 Cellular drivers have the same two separate cases as Wi-Fi. If they use an external IP stack, the driver implements the Network stack API. If they use the internal IP stack, LwIP, then they implement the Serial PPP driver.
 
-##### IEEE 802.15.4 RF driver
+#### IEEE 802.15.4 RF driver
 
 <span class="images">![](https://s3-us-west-2.amazonaws.com/mbed-os-docs-images/rf-driver.png)<span>RF driver</span></span>