docs/block-replication.txt: Fix replication top-id command demo

This demo not correct, the original childs1 can't pass the
the bdrv_is_root_node check in replcation_start().
Keep consistent with docs/COLO-FT.txt

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211018085044.2788276-1-chen.zhang@intel.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

docs/block-replication.txt

@@ -179,7 +179,7 @@ Primary:
 
 Secondary:
   -drive if=none,driver=raw,file.filename=1.raw,id=colo1 \
-  -drive if=none,id=childs1,driver=replication,mode=secondary,top-id=childs1
+  -drive if=none,id=childs1,driver=replication,mode=secondary,top-id=top-disk1
          file.file.filename=active_disk.qcow2,\
          file.driver=qcow2,\
          file.backing.file.filename=hidden_disk.qcow2,\