doc: used ref label for multisite link

doc: used ref label for multisite link

Used the multisite ref label to link the Multi-Site page.
There's no federated config page in the latest docs.
The federated config page seems a little redundant. The multisite page's content
is very similar to the federated config. The multisite page also reflects best practices
when running radosgw's across geographic locations.

Fixes: http://tracker.ceph.com/issues/26997
Signed-off-by: James McClune <jmcclune@mcclunetechnologies.net>