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

Author: Cao Xuan Hoang

doc/source/command-objects/subnet.rst

@@ -151,6 +151,7 @@ List subnets
 
     os subnet list
         [--long]
+        [--ip-version {4,6}]
 
 .. option:: --long