rename zh-cn with zh-hans

According to https://github.com/GitbookIO/gitbook/issues/926, the $lang `zh-cn` have been removed from Gitbook, we should use `zh-hans` instead.
`zh-tw` should be `zh-hant` but gitbook does not support now.
- [Why use the language attribute?](http://www.w3.org/International/questions/qa-lang-why)
- [Language tags in HTML and XML](http://www.w3.org/International/articles/language-tags/#script)
