change Site Language to Simplified Chinese failed.

When I want to change the site language to Simplified Chinese according to the document([Click Here!](http://theme-next.iissnan.com/getting-started.html#select-language)), it tells me I should set the `language` attribute to `zh-Hans` in the site configuration file but I failed. Then I `cd` in the `theme/next/languages/` and find `zh-CN.yml` file in it. So I think should I set the `language` to `zh-CN`, and finally it works!
Please update the document.
I'm appreciate your work about hexo-theme-next. Hope it getting better!

Open the linked Getting Started documentation at the Select Language section and compare its `language` value with the available `zh-CN.yml` file under `theme/next/languages/`. Update the documented setting to the working value, then verify that the example matches the language file name and can be followed successfully.

Written by the indexing model from the issue text.