Fix code-block

Author: damienalexandre

validation/sequence_provider.rst

@@ -143,6 +143,7 @@ that group are valid, the second group, ``Strict``, will be validated.
 You can also define a group sequence in the `validation_groups` form option:
 
 .. code-block:: php
+
     use Symfony\Component\Validator\Constraints\GroupSequence;
     use Symfony\Component\Form\AbstractType;
     // ...