LU-9752 man: Reference zgenhostid instead of genhostid

LU-9752 man: Reference zgenhostid instead of genhostid

In ZFS 0.7.0, they added zgenhostid(8) to be used in place of
Redhat's genhostid, so that there would be a platform agnostic
way to generate /etc/hostid.

Test-Parameters: trivial
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I691266d04f91d5fa7c50b72948c801afa69d647d
Reviewed-on: https://review.whamcloud.com/29327
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>