minor #15185 [Security] Adding typehint (ThomasLandauer)

This PR was submitted for the 4.4 branch but it was merged into the 5.2 branch instead.

Discussion
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[Security] Adding typehint

...from https://symfony.com/doc/current/security/guard_authentication.html#the-guard-authenticator-methods

Commits
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d0d10f6 Adding typehint

Author: javiereguiluz

security/guard_authentication.rst

@@ -187,7 +187,7 @@ This requires you to implement several methods::
             return true;
         }
 
-        public function onAuthenticationSuccess(Request $request, TokenInterface $token, $providerKey)
+        public function onAuthenticationSuccess(Request $request, TokenInterface $token, string $providerKey)
         {
             // on success, let the request continue
             return null;