docs(guide/services): fix link to services api

Closes #7795

Author: fuqcool

docs/content/guide/services.ngdoc

@@ -299,5 +299,5 @@ it('should clear messages after alert', function() {
 
 ## Related API
 
-* {@link ./ng Angular Service API}
+* {@link ./api/ng/service Angular Service API}
 * {@link angular.injector Injector API}