minor #11299 [Flex] Tell that var/logs/ directory was renamed (phansys)

This PR was submitted for the master branch but it was merged into the 4.2 branch instead (closes #11299).

Discussion
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[Flex] Tell that `var/logs/` directory was renamed

Ref: symfony/symfony#28810 (comment)

Commits
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2fcd9bc [Flex] Tell that `var/logs/` directory was renamed

Author: javiereguiluz

setup/flex.rst

@@ -270,6 +270,9 @@ manual steps:
 #. Remove the ``bin/symfony_requirements`` script and if you need a replacement
    for it, use the new `Symfony Requirements Checker`_.
 
+#. Update entry for log directory at ``.gitignore`` and in any other place in order
+   to set ``var/log/``, which previously was ``var/logs/``.
+
 Customizing Flex Paths
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