minor #14410 Fix invalid variable name (apfelbox)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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Fix invalid variable name

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Just a small typo

Commits
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25012da Fix invalid variable name

Author: wouterj

security/guard_authentication.rst

@@ -168,7 +168,7 @@ This requires you to implement several methods::
             // The "username" in this case is the apiToken, see the key `property`
             // of `your_db_provider` in `security.yaml`.
             // If this returns a user, checkCredentials() is called next:
-            return $userProvider->loadUserByUsername($apiToken);
+            return $userProvider->loadUserByUsername($credentials);
         }
 
         public function checkCredentials($credentials, UserInterface $user)