Fixed example of transactional operations in AR guide

docs/guide/db-active-record.md

@@ -969,12 +969,16 @@ as described in "transactions" section of "[Database basics](db-dao.md)". Anothe
 ```php
 class Post extends \yii\db\ActiveRecord
 {
+    public function transactions()
+    {
         return [
             'admin' => self::OP_INSERT,
             'api' => self::OP_INSERT | self::OP_UPDATE | self::OP_DELETE,
             // the above is equivalent to the following:
             // 'api' => self::OP_ALL,
         ];
+    }
+}
 ```
 
 In the above `admin` and `api` are model scenarios and constants starting with `OP_` are operations that should