minor symfony#8263 Add missing comment (janek109)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#8263).

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Add missing comment

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7ab8f51 Add missing comment

Author: xabbuh

forms.rst

@@ -659,6 +659,7 @@ the choice is ultimately up to you.
     good idea to explicitly specify the ``data_class`` option by adding the
     following to your form type class::
 
+        // src/AppBundle/Form/TaskType.php
         use AppBundle\Entity\Task;
         use Symfony\Component\OptionsResolver\OptionsResolver;