minor #17794 [docker] fix composer extra variable name (kaznovac)

This PR was merged into the 5.4 branch.

Discussion
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[docker] fix composer `extra` variable name

fixed transposition in the variable name

this should also be merged to 6.x branches

Commits
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2f8d9d4 [docker] fix composer `extra` variable name

Author: javiereguiluz

setup/docker.rst

@@ -26,7 +26,7 @@ your ``docker-compose.yml`` file will automatically be updated to include a
 
 The first time you install a recipe containing Docker config, Flex will ask you
 if you want to include it. Or, you can set your preference in ``composer.json``,
-by setting the ``symfony.extra.docker`` config to ``true`` or ``false``.
+by setting the ``extra.symfony.docker`` config to ``true`` or ``false``.
 
 Some recipes also include additions to your ``Dockerfile``. To get those changes,
 you need to already have a ``Dockerfile`` at the root of your app *with* the