minor #9982 Update external_resources.rst (deprecated SensioFrameworkExtraBundle's routing annotations) (royklutman)

This PR was submitted for the 2.8 branch but it was merged into the 3.4 branch instead (closes #9982).

Discussion
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Update external_resources.rst (deprecated SensioFrameworkExtraBundle's routing annotations)

Updated because SensioFrameworkExtraBundle's routing annotations have been deprecated.

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Commits
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bfeb600 Update external_resources.rst

Author: javiereguiluz

routing/external_resources.rst

@@ -98,7 +98,7 @@ suppose you want to prefix all routes in the AppBundle with ``/site`` (e.g.
 
     .. code-block:: php-annotations
 
-        use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
+        use Symfony\Component\Routing\Annotation\Route;
 
         /**
          * @Route("/site")