fix FlattenException namespace

Author: alchimik

create_framework/http_kernel_httpkernel_class.rst

@@ -65,7 +65,9 @@ Our code is now much more concise and surprisingly more robust and more
 powerful than ever. For instance, use the built-in ``ExceptionListener`` to
 make your error management configurable::
 
- $errorHandler = function (HttpKernel\Exception\FlattenException $exception) {
+ use Symfony\Component\Debug\Exception\FlattenException;
+ 
+ $errorHandler = function (FlattenException $exception) {
  $msg = 'Something went wrong! ('.$exception->getMessage().')';
 
  return new Response($msg, $exception->getStatusCode());
@@ -88,7 +90,7 @@ The error controller reads as follows::
  namespace Calendar\Controller;
 
  use Symfony\Component\HttpFoundation\Response;
- use Symfony\Component\HttpKernel\Exception\FlattenException;
+ use Symfony\Component\Debug\Exception\FlattenException;
 
  class ErrorController
  {