bsdinstall.8: Fix '---jail' option in manpage

The .Fl macro implicitly adds a '-', so only use a single '-' in the
--jail option.

Fixes: 8d0a90512ee3 ("bsdinstall: Improve pkgbase handling for jails")
MFC after: 1 day
Reported by: Mark Millard <marklmi@yahoo.com>
Sponsored by: https://www.patreon.com/bsdivy