Remove typo in command

JeroenMoonen · web-flow · commit 9545e2adc2bd · 2018-11-15T15:14:26.000+01:00
diff --git a/setup/flex.rst b/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
