rc.conf: Fix typo in comment

The correct path is /etc/defaults/rc.conf (defaults in plural).

Reviewed by: netchild
Fixes: cc4eb1ea1040 ("Add support for a /etc/defaults/vendor.conf override file")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D56456

(cherry picked from commit 8e08080ee581dfd30c128d032035da7b6ae44463)