Update config_system.md

Fix header sizes

Author: Amanda Butler

docs/advanced/config_system.md

@@ -9,7 +9,7 @@ The mbed configuration system customizes the compile time configuration of vario
 
 The configuration system gathers and interprets all the configurations defined in the source tree. The output of the configuration system is a list of macros that are automatically defined when compiling the code.
 
-# Defining configuration parameters
+## Defining configuration parameters
 
 The configuration system understands configuration data defined in targets, libraries and applications. Although there are some slight differences in the way the configuration system works in these cases, the configuration parameters are always defined in a JSON object called "config".
 
@@ -49,7 +49,7 @@ The configuration system automatically appends an *implicit prefix* to the name
 - **app.** if the parameter is defined in the application.
 - The name of the library followed by a dot (.) if the parameter is defined in a library.
 
-# Configuration data in libraries
+## Configuration data in libraries
 
 Each mbed library can have an optional `mbed_lib.json` file located in the root folder of the library that defines its configuration. For a library called `mylib`, the configuration file can look like this:
 
@@ -103,7 +103,7 @@ As explained [here](#defining-configuration-parameters), the parameters have an
 
 If the source tree has code for more than one library, each library needs its own `mbed_lib.json` file in its root folder.
 
-# Configuration data in targets
+## Configuration data in targets
 
 Like libraries, targets can define their own configuration data. Additionally, targets can override the configuration of the target(s) they inherit from (for more details about how do define a target and target inheritance, check [this link](mbed_targets.md)). Target configuration data is defined in `targets.json` using `config`, as described [here](#defining-configuration-parameters). An example for a hypothetical `Base` target is given below:
 
@@ -152,7 +152,7 @@ Targets can inherit from other targets, and their configuration data is also inh
 
 It is an error for a derived target to redefine a configuration parameter already defined by its parent(s) in its `config` section. It is also an error for a derived target to override a configuration parameter that was not defined by its parent(s) in its `overrides` section.
 
-# Configuration data in applications
+## Configuration data in applications
 
 Like target and library configuration, application configuration is optional; if it exists, it must be defined in an `mbed_app.json` file. Unlike library configuration, there can be a single `mbed_app.json` file in the source tree.
 
@@ -203,7 +203,7 @@ Other than this, `target_overrides` works exactly like it does for libraries. Ke
 
 It is an error for the application configuration to override configuration parameters that were not defined.
 
-## Overriding target attributes
+### Overriding target attributes
 
 Target configurations contain a set of attributes that you may manipulate with an application configuration. You may override these attributes as if they were a normal configuration parameter. If these attributes are cumulative, you may also manipulate them with the special `attribute_add` and `attribute_remove` meta-attributes.
 
@@ -226,7 +226,7 @@ For example, this `mbed_app.json` disables the IPV4 feature; using this network
 }
 ```
 
-# Configuration data precedence
+## Configuration data precedence
 
 The order in which the various bits of configurations are considered is:
 
@@ -247,7 +247,7 @@ For `myapp` above:
  	- 128 and 20, respectively, because `Derived` defines the `NXP` label and `mylib` defines a specific configuration for this label. 
 - Because `Derived` has its own `my_own_config` configuration parameter, `target.my_own_config` will also be defined in this case.
 
-# Using configuration data in the code
+## Using configuration data in the code
 
 When compiling, the configuration system will automatically generate macro definitions for the configuration parameters and all the macros defined in libraries and the application in their `macros` keys. These definitions will be written in a file named `mbed_config.h`, located in the build directory. When compiling `myapp` for target `Base`, the `mbed_config.h` file will look like this (note that the order of the definitions may be different):