[Docs] book mode part need to be updated

### What's the problem
https://hackmd.io/how-to-create-book?both#6-Publish
https://hackmd.io/s/how-to-create-book-tw#6-%E4%BB%A5%E6%9B%B8%E6%9C%AC%E6%A8%A1%E5%BC%8F-%E5%85%AC%E9%96%8B%E7%AD%86%E8%A8%98%EF%BC%81
The bookmode button is now residing in `fa-share-alt` button. It would be nice to update the document.
書本模式的發佈按鈕換地方了，說明文件應該配合更新。