Fix Timer.md

Remove class hierarchy diagram, and point Doxygen to .h file.

Author: Amanda Butler

docs/api/drivers/Timer.md

@@ -1,7 +1,5 @@
 ## Timer
 
-<span class="images">![](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/classmbed_1_1_timer.png)<span>Timer class hierarchy</span></span>
-
 Use the Timer interface to create, start, stop and read a timer for measuring small times (between microseconds and seconds).
 
 You can independently create, start and stop any number of Timer objects.
@@ -10,7 +8,7 @@ You can independently create, start and stop any number of Timer objects.
 
 ### Timer class reference
 
-[![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/classmbed_1_1_timer.html)
+[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/_timer_8h_source.html)
 
 ### Timer hello, world