docs: use \ for it is treated as a fully qualified class

Co-authored-by: kenjis <kenji.uui@gmail.com>

Author: datamweb

docs/guides/mobile_apps.md

@@ -11,7 +11,7 @@ Start by creating a route that would handle the request from the login screen on
 ```php
 
 // Routes.php
-$routes->post('auth/token', 'App\Controllers\Auth\LoginController::mobileLogin');
+$routes->post('auth/token', '\App\Controllers\Auth\LoginController::mobileLogin');
 
 // LoginController.php
 namespace App\Controllers\Auth;