Mentioned APP_ENV for Flex apps

Author: javiereguiluz

deployment.rst

@@ -156,8 +156,9 @@ as you normally do:
 .. caution::
 
     If you get a "class not found" error during this step, you may need to
-    run ``export APP_ENV=prod`` before running this command so that
-    the ``post-install-cmd`` scripts run in the ``prod`` environment.
+    run ``export SYMFONY_ENV=prod`` (or ``export APP_ENV=prod`` if you're
+    using :doc:`Symfony Flex </setup/flex>`) before running this command so
+    that the ``post-install-cmd`` scripts run in the ``prod`` environment.
 
 D) Clear your Symfony Cache
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~