minor #11614 doctrine/doctrine-fixtures-bundle is a dev req (juanmiguelbesada)

This PR was merged into the 3.4 branch.

Discussion
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doctrine/doctrine-fixtures-bundle is a dev req

Update the command used to install doctrine/doctrine-fixtures-bundle to install it as a development requirement, and not an app requirement.

This command should be updated in branches 3.4, 4.2 and master, but I don't know if it is done automatically.

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Commits
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2a2f402 doctrine/doctrine-fixtures-bundle is a dev-req

Author: javiereguiluz

best_practices/business-logic.rst

@@ -309,7 +309,7 @@ the following command to install the Doctrine fixtures bundle:
 
 .. code-block:: terminal
 
-    $ composer require doctrine/doctrine-fixtures-bundle
+    $ composer require --dev doctrine/doctrine-fixtures-bundle
 
 Then, enable the bundle in ``AppKernel.php``, but only for the ``dev`` and
 ``test`` environments::