Update Behaviors.rst

Add "use Cake\ORM\Query" to "Defining Event Listeners" example code

Prevents from : 
Warning (4096): Argument 1 passed to App\Model\Behavior\SluggableBehavior::findSlug() must be an instance of App\Model\Behavior\Query, instance of Cake\ORM\Query given [APP/Model/Behavior/SluggableBehavior.php, line 23]