docs/169317: zfs umount refers to umount(1M) but should to umount(8)

[eadler at FreeBSD.org]

Synopsis: zfs umount refers to umount(1M) but should to umount(8)

Responsible-Changed-From-To: eadler->freebsd-doc
Responsible-Changed-By: eadler
Responsible-Changed-When: Sun Sep 2 16:52:53 UTC 2012
Responsible-Changed-Why: 
I won't be looking at this PR for a while and I need to clear some out
of my queue

http://www.freebsd.org/cgi/query-pr.cgi?pr=169317