doc: clarify fs.utimes() arguments

Make it clear that atime and mtime should be in seconds.

PR-URL: https://github.com/nodejs/node/pull/8651


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>