sbin/devfs: Correct usage

The -s applies to rule keyword only and it follows the rule keyword.

MFC after: 1 week
PR: 263289
Submitted by: Yuichiro NAITO <naito.yuichiro@gmail.com>
Fixes: c3e412c08333 sbin/devfs: clarify usage
Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D34934