Update controller.rst

Invalid flash example for php template leading to::

FatalErrorException: Error: Call to undefined method Symfony\Bundle\FrameworkBundle\Templating\Helper\SessionHelper::getFlashBag()

Author: dczech

book/controller.rst

@@ -721,7 +721,7 @@ the ``notice`` message:
 
     .. code-block:: html+php
 
-        <?php foreach ($view['session']->getFlashBag()->get('notice') as $message): ?>
+        <?php foreach ($view['session']->getFlash('notice') as $message): ?>
             <div class="flash-notice">
                 <?php echo "<div class='flash-error'>$message</div>" ?>
             </div>