Update mailer.rst

Needs Annotation\Route in the use section for the route to be detected.

Author: mark2016

mailer.rst

@@ -284,6 +284,7 @@ and create an :class:`Symfony\\Component\\Mime\\Email` object::
 
     use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
     use Symfony\Component\HttpFoundation\Response;
+    use Symfony\Component\Routing\Annotation\Route;
     use Symfony\Component\Mailer\MailerInterface;
     use Symfony\Component\Mime\Email;