minor symfony#8056 Fix variable name in CSFR token example code (rubenrubiob)

This PR was merged into the 3.3 branch.

Discussion
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Fix variable name in CSFR token example code

Commits
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7b85999 Fix variable name in CSFR token example code

Author: xabbuh

security/guard_authentication.rst

@@ -502,7 +502,7 @@ and add the following logic::
 
         public function getCredentials(Request $request)
         {
-            $token = $request->request->get('_csrf_token');
+            $csrfToken = $request->request->get('_csrf_token');
             
             if (false === $this->csrfTokenManager->isTokenValid(new CsrfToken('authenticate', $csrfToken))) {
                 throw new InvalidCsrfTokenException('Invalid CSRF token.');