bug #3414 add missing code-block directive (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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add missing code-block directive

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

9b1aab8 add missing code-block directive

reference/forms/types/options/data_class.rst.inc

@@ -9,6 +9,8 @@ data_class
 This option is used to set the appropriate data mapper to be used by the form,
 so you can use it for any form field type which requires an object.
 
+.. code-block:: php
+
     $builder->add('media', 'sonata_media_type', array(
         'data_class' => 'Acme\DemoBundle\Entity\Media',
     ));