ifconfig: Minor documentation fix

Fix what appears to have been a small copy/paste typo in ifconfig(8)'s
documentation (man page and header file).

Not that it matters anymore.

Reference: Table I-2 in IEEE Std 802.1Q-2014.

PR: 255557
Submitted by: Jose Luis Duran <jlduran@gmail.com>
MFC after: 1 week