nvmecontrol: Fix minor usage typo

PR: 282930
MFC after: 3 days
Reported by: Gian-Simon Purkert <gspurki@gmail.com>
Reviewed by: imp, jhb, emaste
Differential Revision: https://reviews.freebsd.org/D47726

(cherry picked from commit e7aec3ccf759dcd3fe56ae2bdbec92839c166160)