Update error_pages.rst

This appears to be a typo, as showException gives an error on Symfony 4.2 that reports confusion between showAction and showException.  Setting the twig config to showAction fixes the problem and displays the correct result.

Author: brandinchiu

controller/error_pages.rst

@@ -213,7 +213,7 @@ configuration option to point to it:
 
         # config/packages/twig.yaml
         twig:
-            exception_controller: App\Controller\ExceptionController::showException
+            exception_controller: App\Controller\ExceptionController::showAction
 
     .. code-block:: xml