doc/rgw: fix caption

Rewrite the third paragraph of the caption to the figure depicting how a
zonegroup works.

Follows ceph#50316.

Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 8fa5214)

doc/radosgw/multisite.rst

@@ -77,8 +77,8 @@ movement of data from the applications to the zonegroup. The arrow from the
 master zone (US-EAST) to the secondary zone (US-WEST) represents an act of data
 synchronization.
 
-At the bottom of this diagram, we see the master zonegroup that contains the
-master zone and the secondary zone.
+At the bottom of this diagram, we see the data distributed into the Ceph
+Storage Cluster.
 
 For additional details on setting up a cluster, see `Ceph Object Gateway for
 Production <https://access.redhat.com/documentation/en-us/red_hat_ceph_storage/3/html/ceph_object_gateway_for_production/index/>`__.