minor #18366 Update flex_private_recipes.rst (alamirault)

This PR was merged into the 5.4 branch.

Discussion
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Update flex_private_recipes.rst

BC anchor is broken on https://symfony.com/doc/current/setup/flex_private_recipes.html, my bad !

Commits
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ba4d99e Update flex_private_recipes.rst

Author: javiereguiluz

setup/flex_private_recipes.rst

@@ -16,7 +16,7 @@ perform their own installation tasks. To do this, you need to complete several s
 * Configure your project's ``composer.json`` file; and
 * Install the recipes in your project.
 
-.. _create-a-private-github-repository
+.. _create-a-private-github-repository:
 
 Create a Private Repository
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