minor #8307 Update deployment.rst (proArtex, javiereguiluz)

This PR was merged into the 3.3 branch.

Discussion
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Update deployment.rst

Looks like SYMFONY_ENV is no longer exists (flex + framework-bundle). It is now APP_ENV

Commits
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8e072da Mentioned APP_ENV for Flex apps
0eee3f1 Update deployment.rst

Author: javiereguiluz

deployment.rst

@@ -159,8 +159,9 @@ as you normally do:
 .. caution::
 
     If you get a "class not found" error during this step, you may need to
-    run ``export SYMFONY_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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~