minor symfony#11146 remove .. code-block:: php (OskarStark)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#11146).

Discussion
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remove .. code-block:: php

and replace it by a text

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Commits
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b9edc20 remove .. code-block:: php

Author: javiereguiluz

form/events.rst

@@ -339,7 +339,7 @@ Event subscribers have different uses:
 * Listening to multiple events;
 * Regrouping multiple listeners inside a single class.
 
-.. code-block:: php
+Consider the following example of an ``AddEmailFieldListener``::
 
     // src/AppBundle/Form/EventListener/AddEmailFieldListener.php
     namespace AppBundle\Form\EventListener;