Added php opening tag to sample controller

It took me a while to figure out why the sample didn't seem to work, despite being experimented in php ; i checked many times namespace, uses and class declaration before finding that i just missed the opening tag. Think it would help others (even more if unexperimented) discovering symfony by copy/pasting sample code from the doc !

Author: Krap

page_creation.rst

@@ -45,6 +45,7 @@ random) number and prints it. To do that, create a "Controller class" and a
 "controller" method inside of it that will be executed when someone goes to
 ``/lucky/number``::
 
+    <?php
     // src/AppBundle/Controller/LuckyController.php
     namespace AppBundle\Controller;