docs: fix a typo on the description of "acl check permissions"

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>

Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Sep  2 23:03:01 CEST 2013 on sn-devel-104