minor changes to ticker doc

Author: aashishc1988

docs/api/drivers/Ticker.md

@@ -1,6 +1,8 @@
 ## Ticker
 
-<span class="images">![](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/classmbed_1_1_ticker.png)<span>Ticker class hierarchy</span></span>
+### Ticker class hierarchy
+
+<span class="images">![](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/classmbed_1_1_ticker.png)</span>
 
 Use the Ticker interface to set up a recurring interrupt; it calls a function repeatedly and at a specified rate.
 
@@ -12,7 +14,7 @@ You can create any number of Ticker objects, allowing multiple outstanding inter
 
 - No printf, malloc or new in ISR: avoid any call to bulky library functions. In particular, certain library functions (such as printf, malloc and new) are not re-entrant, and their behavior could be corrupted when called from an ISR.
 
-- While an event is attached to a Ticker, deep sleep is blocked to maintain accurate timing. If you don't need microsecond precision, consider using the LowPowerTicker class instead because this does not block deep sleep mode.
+- While an event is attached to a Ticker, deep sleep is blocked to maintain accurate timing. If you don't need microsecond precision, consider using the LowPowerTicker class instead because that does not block deep sleep mode.
 
 ### Ticker class reference