minor #14820 Update access_denied_handler.rst (alihasana)

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

Discussion
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Update access_denied_handler.rst

getException is deprecated from symfony 4.4,  as per event listerner docs we have to use getThrowable()

Commits
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03611c5 Update access_denied_handler.rst

Author: OskarStark

security/access_denied_handler.rst

@@ -210,7 +210,7 @@ configure a :ref:`kernel.exception listener <use-kernel-exception-event>`::
 
         public function onKernelException(ExceptionEvent $event): void
         {
-            $exception = $event->getException();
+            $exception = $event->getThrowable();
             if (!$exception instanceof AccessDeniedException) {
                 return;
             }