MFV r275547:

Port Illumos 'zfs allow' examples update.  While I'm there also fix
a typo.

Illumos issue:
    4181 zfs(1m): 'zfs allow' examples in the man page are outdated

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275739 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f