Fix link in DeviceKey.md

Change class reference link to point to class reference instead of .h file.

Author: Amanda Butler

docs/reference/api/drivers/Devicekey.md

@@ -47,7 +47,7 @@ Run the DeviceKey functionality test with the `mbed` command as following:
 
 ### DeviceKey API class reference
 
-[![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/nvstore_8h_source.html)
+[![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_device_key.html)
 
 ### DeviceKey example