minor symfony#9389 Update json_login_setup.rst (CurvesFR)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#9389).

Discussion
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Update json_login_setup.rst

Commits
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11b9f81 Update json_login_setup.rst

Author: javiereguiluz

security/json_login_setup.rst

@@ -69,6 +69,7 @@ path:
         // src/AppBundle/Controller/SecurityController.php
 
         // ...
+        use Symfony\Bundle\FrameworkBundle\Controller\Controller;
         use Symfony\Component\HttpFoundation\Request;
         use Symfony\Component\Routing\Annotation\Route;