Fixed error in missing translations example

docs/guide/tutorial-i18n.md

@@ -463,7 +463,7 @@ use yii\i18n\MissingTranslationEvent;
 
 class TranslationEventHandler
 {
-    public static function(MissingTranslationEvent $event) {
+    public static function handleMissingTranslation(MissingTranslationEvent $event) {
         $event->translatedMessage = "@MISSING: {$event->category}.{$event->message} FOR LANGUAGE {$event->language} @";
     }
 }