Add link to itm.md

Add missing link.

Author: Amanda Butler

docs/reference/contributing/target/itm.md

@@ -24,6 +24,8 @@ Some SWO viewers do not allow an arbitrary frequency to be set. Make sure that t
 
 #### Implementing the ITM API
 
+[![View code](https://www.mbed.com/embed/?type=library)](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/group__itm__hal.html)
+
 - You must implement the function `itm_init`. When the function is called:
   - The function must initialize the debug clock for the ITM.
   - The function must configure the SWO pin for debug output.