[placement] Update user doc with api-ref link
[placement] Update user doc with api-ref link

The Placement API user guide [1] mentions a not-yet-available REST API
reference which has since become available [2]; update with the link.

[1] https://docs.openstack.org/nova/pike/user/placement.html#api-reference
[2] https://developer.openstack.org/api-ref/placement/

Closes-Bug: #1714284
Change-Id: Ic368327aa4a81f4217aecb4af3d9358502676234
(cherry picked from commit 4784909352dd1cbb21ba12ec39a863f1ad5ed746)

vmturbo/nova,vmturbo/nova,vmturbo/nova,vmturbo/nova