Update form_collections.rst

i fix an indent error. I hope it was the last one !

form/form_collections.rst

@@ -750,7 +750,7 @@ the relationship between the removed ``Tag`` and ``Task`` object.
  You can optionnally reduce the code in your controller and make it reusable to handle other embedded form.
  First create a helper :
 
- // App\Helper
+  // App\Helper
  use Doctrine\Common\Collections\ArrayCollection;
  use Doctrine\ORM\EntityManagerInterface;