add missing code-block directive

Author: xabbuh

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',
     ));