bug #3871 Fix missing Front Controller (parthasarathigk)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3871).

Discussion
----------

Fix missing Front Controller

Added `app_dev.php` front controller to the demo url.

Commits
-------

f2df407 Fix missing Front Controller

quick_tour/the_big_picture.rst

@@ -84,7 +84,7 @@ main concepts. Go to the following URL to be greeted by Symfony2 (replace
 
 .. code-block:: text
 
-    http://localhost:8000/demo/hello/Fabien
+    http://localhost:8000/app_dev.php/demo/hello/Fabien
 
 .. image:: /images/quick_tour/hello_fabien.png
    :align: center