minor #16585 [Mailer] Update mailer.rst (mark2016)

This PR was submitted for the 6.0 branch but it was squashed and merged into the 4.4 branch instead.

Discussion
----------

[Mailer] Update mailer.rst

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

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `6.x` for features of unreleased versions).

-->

Commits
-------

a7ac41f [Mailer] Update mailer.rst

Author: javiereguiluz

mailer.rst

@@ -229,6 +229,7 @@ and create an :class:`Symfony\\Component\\Mime\\Email` object::
     use Symfony\Component\HttpFoundation\Response;
     use Symfony\Component\Mailer\MailerInterface;
     use Symfony\Component\Mime\Email;
+    use Symfony\Component\Routing\Annotation\Route;
 
     class MailerController extends AbstractController
     {