docs: fix example code of extending controller

Author: datamweb

docs/customization.md

@@ -133,10 +133,11 @@ updates that might happen in the controllers.
 namespace App\Controllers;
 
 use CodeIgniter\Shield\Controllers\LoginController as ShieldLogin;
+use CodeIgniter\HTTP\RedirectResponse;
 
 class LoginController extends ShieldLogin
 {
-    public function logoutAction()
+    public function logoutAction(): RedirectResponse
     {
         // new functionality
     }