Adding typehint

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

Author: ThomasLandauer

security/guard_authentication.rst

@@ -182,7 +182,7 @@ 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;