minor #10678 Remove typo in command (JeroenMoonen)

This PR was merged into the 4.1 branch.

Discussion
----------

Remove typo in command

Remove typo in command

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

9545e2a Remove typo in command

Author: javiereguiluz

setup/flex.rst

@@ -206,7 +206,7 @@ manual steps:
 
    .. code-block:: terminal
 
-       $ rm -fr vendor/*
+       $ rm -rf vendor/*
        $ composer install
 
 #. No matter which of the previous steps you followed. At this point, you'll have