Add --ip-version filtering option to subnet.rst
Add --ip-version filtering option to subnet.rst

When executed "openstack subnet list --help" we can see this is
supported for --ip-version filtering option.
But this option is missing in the rst document.

Change-Id: Ie5443f1da086a1ad455fbeaa848b50a0d9d4b290
Closes-Bug: #1610872

openstack/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,dtroyer/python-openstackclient