Add zsh completion for 'docker service {create,update} --registry-auth'

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -1076,6 +1076,7 @@ __docker_service_subcommand() {
         "($help)--name=[Service name]:name: "
         "($help)*--network=[Network attachments]:network: "
         "($help)*"{-p=,--publish=}"[Publish a port as a node port]:port: "
+        "($help)--registry-auth[Send registry authentication details to Swarm agents]"
         "($help)--replicas=[Number of tasks]:replicas: "
         "($help)--reserve-cpu=[Reserve CPUs]:value: "
         "($help)--reserve-memory=[Reserve Memory]:value: "