fixup! Callback Validation Constraint: Remove reference to deprecated option

Author: Éric

reference/constraints/Callback.rst

@@ -255,6 +255,10 @@ constructor of the Callback constraint::
     // src/AppBundle/Entity/Author.php
     namespace AppBundle\Entity;
 
+    use Symfony\Component\Validator\Context\ExecutionContextInterface;
+    // if you're using the older 2.4 validation API, you'll need this instead
+    // use Symfony\Component\Validator\ExecutionContextInterface;
+
     use Symfony\Component\Validator\Mapping\ClassMetadata;
     use Symfony\Component\Validator\Constraints as Assert;