Add caption to Ticker.md

Add class hierarchy caption.

Author: Amanda Butler

docs/api/drivers/Ticker.md

@@ -2,7 +2,7 @@
 
 ### 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>
+<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>
 
 Use the Ticker interface to set up a recurring interrupt; it calls a function repeatedly and at a specified rate.