Chinese version of API doc needs to be updated to reflect set/getBucketPolicy fix (PR#639)

[ebozduman]

For issue #635 (PR#639), we have to modify the Chinese version of the API document, ../docs/zh_CN/API.md
All the references to Policy.READ_ONLY，Policy.WRITE_ONLY，Policy.READ_WRITE, Policy.NONE, and set_bucket_policy and get_bucket_policy signatures (removal of prefix) need to be modified/replaced.

[deekoder]

I have pinged @dingjs for this.