docs: fix sample code

Co-authored-by: John Paul E. Balandan, CPA <51850998+paulbalandan@users.noreply.github.com>

Author: kenjis

user_guide_src/source/incoming/routing.rst

@@ -261,7 +261,7 @@ config options like namespace, subdomain, etc. Without necessarily needing to ad
 an empty string in place of the prefix and the routes in the group will be routed as though the group never existed but with the
 given route config options::
 
-    $routes->group('', ['namespace' => 'Myth\Auth\Controllers'], function ($routes) {
+    $routes->group('', ['namespace' => 'Myth\Auth\Controllers'], static function ($routes) {
         $routes->get('login', 'AuthController::login', ['as' => 'login']);
         $routes->post('login', 'AuthController::attemptLogin');
         $routes->get('logout', 'AuthController::logout');