manpages: prefer linking to xo_options.7 over xo_parse_args.3

Most libxo-enabled utilities link to xo_parse_args.3. But that man page
doesn't actually describe how to format command-line arguments. Rather,
xo_options.7 does. I suspect that these utilities link to the former
because the latter man page did not exist until libxo-0.7.2
(8a6eceff3ce). And newer utilities have probably been copy/pasting the
same text from older ones. But the superior man page does exist now,
so we should link to it instead.

MFC after: 2 weeks
Sponsored by: ConnectWise
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D51354