service(8): Fix typo in man page.
Differential revision: https://reviews.freebsd.org/D32582
Submitted by: christos@
Sponsored by: NVIDIA Networking
(cherry picked from commit 66d795ec191b994f317b0ad812bb6e7943347c1f)
(cherry picked from commit 808108da324eee5d777134317c3492058d0fe964)