minor #4647 Update controllers.rst (keefekwan)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4647).

Discussion
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Update controllers.rst

Added in Route/ParamConverter use statements and @ParamConverter annotation

Commits
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1a0534d Update controllers.rst

Author: wouterj

best_practices/controllers.rst

@@ -136,6 +136,8 @@ For example:
 
 .. code-block:: php
 
+    use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
+    
     /**
      * @Route("/{id}", name="admin_post_show")
      */