dmesg.8: Index kern.msgbuf_show_timestamp

This is the only place this important debugging tunable is documented.
Mark it up with the Va macro according to style.mdoc(5) so that people
can find it via apropos Va=kern.msg, the standard syntax to search
the FreeBSD manual for sysctls and tunables.

Fixes: 6910fee62e77 (dmesg: Document kern.msgbuf_show_timestamp)