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

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

contrib/completion/zsh/_docker

@@ -1768,6 +1768,7 @@ __docker_service_subcommand() {
         "($help)--restart-delay=[Delay between restart attempts]:delay: "
         "($help)--restart-max-attempts=[Maximum number of restarts before giving up]:max-attempts: "
         "($help)--restart-window=[Window used to evaluate the restart policy]:window: "
+        "($help)*--secret=[Specify secrets to expose to the service]:secret:__docker_complete_secrets"
         "($help)--stop-grace-period=[Time to wait before force killing a container]:grace period: "
         "($help -t --tty)"{-t,--tty}"[Allocate a pseudo-TTY]"
         "($help)--update-delay=[Delay between updates]:delay: "