Update ActiveForm.php

Incorrect character for generating docs. See http://www.yiiframework.com/doc-2.0/yii-bootstrap-activeform.html

extensions/bootstrap/ActiveForm.php

@@ -21,7 +21,7 @@ use yii\base\InvalidConfigException;
  * use yii\bootstrap\ActiveForm;
  *
  * $form = ActiveForm::begin(['layout' => 'horizontal'])
- *  ```
+ * ```
  *
  * This will set default values for the [[yii\bootstrap\ActiveField|ActiveField]]
  * to render horizontal form fields. In particular the [[yii\bootstrap\ActiveField::template|template]]