doc: fix type of atime/mtime

It seems that fs.unlinkSync and fs.utimesSync can accept number, Date or string as same as fs.unlink and fs.utimes.

https://github.com/nodejs/node/blob/3a2e75d9a5c31d20e429d505b82dd182e33f459a/lib/fs.js#L1177-L1196

https://github.com/nodejs/node/blob/3a2e75d9a5c31d20e429d505b82dd182e33f459a/lib/fs.js#L1189-L1206

Checklist

• documentation is changed or added
• commit message follows commit guidelines