Replace Doxygen link in UARTService.md

Replace Doxygen link with .h file because class page doesn't exist anymore and is breaking the build.

Author: Amanda Butler

docs/api/bluetooth/UARTService.md

@@ -4,7 +4,7 @@
 
 ### UARTService class reference
 
-[![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/development/mbed-os-api-doxy/class_u_a_r_t_service.html)
+[![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/mbed-os/development/mbed-os-api-doxy/_u_a_r_t_service_8h_source.html)
 
 ### UARTService example