Add link to rtc.md

Add missing link.

Author: Amanda Butler

docs/contributing/rtc.md

@@ -24,7 +24,7 @@ The RTC HAL API provides a low-level interface to the Real Time Counter (RTC) of
 
 ### Implementing the RTC API
 
-RTC HAL API is in [hal/rtc_api.h](). You need to implement the following functions to support RTC:
+RTC HAL API is in [hal/rtc_api.h](/docs/v5.4/mbed-os-api-doxy/rtc__api_8h_source.html). You need to implement the following functions to support RTC:
 
 To enable sleep support in Mbed OS, you need to add the `RTC` label in the `device_has` option of the target's section in the `targets.json` file.