1492: fix bad link within doc (#1497)

Author: ChrisGooch-Arm

docs/api/platform/platform.md

@@ -39,7 +39,7 @@ For example, the following code allows a user to provide a customized response w
 
 [![View code](https://www.mbed.com/embed/?url=https://github.com/ARMmbed/mbed-os-snippet-PlatfromOverview_Callbacks/tree/v6.7)](https://github.com/ARMmbed/mbed-os-snippet-PlatfromOverview_Callbacks/blob/v6.7/main.cpp)
 
-The Callback class manages C/C++ function pointers so you don't have to. If you are asking yourself why you should use the Callback class, you should read the [Importance of State](platform.html#the-importance-of-state) section.
+The Callback class manages C/C++ function pointers so you don't have to. If you are asking yourself why you should use the Callback class, you should read the [Importance of State](platform-concepts.html#the-importance-of-state) section.
 
 #### Why should you use Callbacks?