packages: Move kernel-man to the minimal set

Currently this is in the "kernels" set, which doesn't make sense
because it's not a kernel.

Move it to the "minimal" set, on the basis that minimal already includes
a bunch of manpages, and a user who installs with minimal expects that
e.g. "man ix" will work.

MFC after: 3 seconds
Reported by: des
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D52560

(cherry picked from commit a2783fb75401c85f5ae815a1349fbad8e7ab7b3c)