doc: fix type of atime/mtime of fs.unlinkSync

It seems that fs.unlinkSync accepts number, Date or string as same as fs.unlink https://github.com/nodejs/node/blob/3a2e75d9a5c31d20e429d505b82dd182e33f459a/lib/fs.js#L1177-L1196

Checklist

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