bridge.4: Fix the synopsis for -ifuntagged

This command doesn't accept a vlan-id.

PR: 290141
MFC after: 3 days
Reported by: Paul Procacci <pprocacci@gmail.com>
Reviewed by: ziaee (manpages)
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53049

(cherry picked from commit dc978c5cb55b75d292c7ef32564f5c376685234b)