mfi: correct typo in sysctl/tunable description
Reported by: jrtc27
Reviewed by: lwhsu
Sponsored by: The FreeBSD Foundation
Event: Kitchener-Waterloo Hackathon 202406
(cherry picked from commit e0dcb9b611d3b3e471d7bc30bcff7cd4e26c6f19)
(cherry picked from commit 9ebdfc41bd34c355152e554151c92e74fc2f1af0)