netmap: Correct a comment
Reviewed by: vmaffione
MFC after: 1 week
Sponsored by: Zenarmor
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38063
(cherry picked from commit 854b2f302dcc73dfef2e4458d382f8149ce9167c)