Minor tweaks to zfs.8 man page for POSIX ACLs

Minor tweaks to zfs.8 man page for POSIX ACLs

* Capitalize POSIX in POSIX ACLs.  This change makes the POSIX
  in POSIX ACLs all caps, which is both correct and consistent with
  the rest of the man page.

* Slightly reword part of zfs.8.  I tweaked a sentence to add a
  missing comma, and as long as I was editing, removed a couple
  unnecessary words.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8220