bug symfony#8739 Fix upgrading to Flex (michalpipa)

This PR was merged into the 3.3 branch.

Discussion
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Fix upgrading to Flex

When upgrading to Flex `composer install` won't install any new (merged) dependencies. `composer update` should be used instead.

Commits
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91da8a2 Fix upgrading to Flex

Author: weaverryan

setup/flex.rst

@@ -163,7 +163,7 @@ manual steps:
    new project (don't copy the ``symfony/symfony`` dependency, but add the
    relevant components you are effectively using in your project).
 
-#. Install the dependencies in the new project executing ``composer install``.
+#. Install the dependencies in the new project executing ``composer update``.
    This will make Symfony Flex generate all the configuration files in
    ``config/packages/``