Update auth.rst

Type hinting related warning like:

```php
Strict (2048): Declaration of App\Controller\AppController::beforeFilter() should be compatible with Cake\Controller\Controller::beforeFilter(Cake\Event\Event $event) [APP/Controller/AppController.php, line 34]

Warning (4096): Argument 1 passed to Cake\Controller\Controller::beforeFilter() must be an instance of Cake\Event\Event, null given, called in /home/totofriends/apps/app/App/Controller/AppController.php on line 67 and defined [CORE/src/Controller/Controller.php, line 663]
```
Merge pull request #1450 from luke83/patch-2

Update auth.rst