fixed note about post redirect get

Fixed guide example by adding note about redirect / refresh . See this [comments](https://github.com/yiisoft/yii2/commit/0e18fbf89331305d5caed7ae5aafe176154bc1d3#commitcomment-6620020) for more info .
https://github.com/yiisoft/yii2/blob/master/framework/web/Controller.php#L143 and https://github.com/yiisoft/yii2/blob/master/framework/web/Controller.php#L201. Note that you need to use `[[` syntax as in other docs.