OpenZFS 9763 - zfs(1M): broken formatting in allow/unallow description

OpenZFS 9763 -  zfs(1M): broken formatting in allow/unallow description

Porting notes:
* Two of the three changes from the upstream patch were already
  applied for Linux.  Only the last one is required.

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Approved by: Gordon Ross <gwr@nexenta.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://illumos.org/issues/9763
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8a702e55
Closes #7972