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 .
In general it should be in separate commits but since you've started on it, yes, please verify that there were no other changes since latest translation using tools in that guide.