Update i18n.md

Correction name ru_RU zh_CN to ru-RU zh-CN

docs/guide/i18n.md

@@ -27,13 +27,13 @@ Target language is what's currently used. It's defined in application configurat
 return [
 	'id' => 'applicationID',
 	'basePath' => dirname(__DIR__),
-	'language' => 'ru_RU' // ← here!
+	'language' => 'ru-RU' // ← here!
 ```
 
 Later you can easily change it in runtime:
 
 ```php
-\Yii::$app->language = 'zh_CN';
+\Yii::$app->language = 'zh-CN';
 ```
 
 > **Note**: please refer to [ICU documentation](http://userguide.icu-project.org/locale) in order to find out language