bug #3541 Update generic_event.rst (Lumbendil)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3541).

Discussion
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Update generic_event.rst

The $event['data'] field was not set, thus it wouldn't be modified.

Also, the example already had a lowercase string. Uppercased 1 letter.

Commits
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029f4e1 Update generic_event.rst

components/event_dispatcher/generic_event.rst

@@ -93,7 +93,7 @@ Filtering data::
 
     use Symfony\Component\EventDispatcher\GenericEvent;
 
-    $event = new GenericEvent($subject, array('data' => 'foo'));
+    $event = new GenericEvent($subject, array('data' => 'Foo'));
     $dispatcher->dispatch('foo', $event);
 
     echo $event['data'];
@@ -102,6 +102,6 @@ Filtering data::
     {
         public function filter(GenericEvent $event)
         {
-            strtolower($event['data']);
+            $event['data'] = strtolower($event['data']);
         }
     }