volumegroup: correct comment got the ControllerGetVolumeGroup RPC call

The comment mentions CreateVolumeGroup, which should be
ControllerGetVolumeGroup.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

volumegroup/README.md

@@ -32,7 +32,7 @@ service Controller {
   // ListVolumeGroups RPC call to list volume groups.
   rpc ListVolumeGroups(ListVolumeGroupsRequest)
       returns (ListVolumeGroupsResponse) {}
-  // CreateVolumeGroup RPC call to get a volume group.
+  // ControllerGetVolumeGroup RPC call to get a volume group.
   rpc ControllerGetVolumeGroup(ControllerGetVolumeGroupRequest)
       returns (ControllerGetVolumeGroupResponse) {}
 }