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
     {