minor #4219 Cache needs be cleared (burki94)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4219).

Discussion
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Cache needs be cleared

The configuration is cached, so running assetic:dump before clearing the cache won't use the new configuration.

Commits
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7c2c17f Update override_dir_structure.rst
dee483f Cache needs be cleared

cookbook/configuration/override_dir_structure.rst

@@ -147,9 +147,10 @@ the ``extra.symfony-web-dir`` option in the ``composer.json`` file:
                 'read_from' => '%kernel.root_dir%/../../public_html',
             ));
 
-    Now you just need to dump the assets again and your application should
+    Now you just need to clear the cache and dump the assets again and your application should
     work:
 
     .. code-block:: bash
-
+    
+        $ php app/console cache:clear --env=prod
         $ php app/console assetic:dump --env=prod --no-debug