minor #9137 Fixed wrong variable name (kevin-lot)

This PR was merged into the 4.0 branch.

Discussion
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Fixed wrong variable name

Replaced by good environment variable name.

Commits
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8b03c42 Fixed wrong variable name

Author: javiereguiluz

configuration/environments.rst

@@ -131,7 +131,7 @@ always run the application in the dev environment, independent of the
     argument to the ``Kernel`` constructor. This specifies if the application
     should run in "debug mode". Regardless of the environment, a Symfony
     application can be run with debug mode set to ``true`` or ``false``
-    (respectively ``1`` or ``0`` for the ``APP_ENV`` variable defined in
+    (respectively ``1`` or ``0`` for the ``APP_DEBUG`` variable defined in
     ``.env``). This affects many things in the application, such as displaying
     stacktraces on error pages or if cache files are dynamically rebuilt on
     each request.  Though not a requirement, debug mode is generally set to