sbin/devfs: Correct usage

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

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

(cherry picked from commit f9f42a709b2b3f8c8d3de517f301d4ccb33e7b16)