docs: fix registry mirror example

The docker configuration should be a URL, not a host:port.

Closes-Bug: #1919932
Change-Id: I5025fdb7e48c79a107b45f1454f5d5e81367a2f9

doc/source/user/multinode.rst

@@ -75,7 +75,7 @@ IP address and port on which the registry is listening:
 
    docker_custom_config:
      registry-mirrors:
-       - 192.168.1.100:4000
+       - http://192.168.1.100:4000
 
 .. _edit-inventory: