Update api-ref location

The api documentation is now published on docs.openstack.org instead
of developer.openstack.org. Update all links that are changed to the
new location.

Note that redirects will be set up as well but let's point now to the
new location.

For details, see:
http://lists.openstack.org/pipermail/openstack-discuss/2019-July/007828.html

Change-Id: I111561b6081306ef5069140a6d00252ee04c7efb

README.rst

@@ -17,7 +17,7 @@ organized and maintain the status of the backups.
 To learn how to use Freezer's API, consult the documentation available online
 at:
 
-- `Backup API Reference <https://developer.openstack.org/api-ref/backup/>`__
+- `Backup API Reference <https://docs.openstack.org/api-ref/backup/>`__
 
 For more information on OpenStack APIs, SDKs and CLIs in general, refer to: