minor #15919 Fixed a grammar issue in flex.rst (flyingfisch)

This PR was merged into the 5.3 branch.

Discussion
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Fixed a grammar issue in flex.rst

Fixed a grammar issue and slightly changed the wording of part of flex.rst to hopefully flow better.

Commits
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292db45 Fixed a grammar issue in flex.rst

Author: javiereguiluz

setup/flex.rst

@@ -89,7 +89,7 @@ manual steps:
        $ rm -rf vendor/*
        $ composer install
 
-#. No matter which of the previous steps you followed. At this point, you'll have
+#. Regardless of which of the previous steps you followed, at this point you'll have
    lots of new config files in ``config/``. They contain the default config
    defined by Symfony, so you must check your original files in ``app/config/``
    and make the needed changes in the new files. Flex config doesn't use suffixes