Add zsh completion for 'docker service ls --filter mode'

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

contrib/completion/zsh/_docker

@@ -1762,6 +1762,10 @@ __docker_service_complete_ls_filters() {
             (id)
                 __docker_complete_services_ids && ret=0
                 ;;
+            (mode)
+                opts=('global' 'replicated')
+                _describe -t mode-opts "mode options" opts && ret=0
+                ;;
             (name)
                 __docker_complete_services_names && ret=0
                 ;;
@@ -1770,7 +1774,7 @@ __docker_service_complete_ls_filters() {
                 ;;
         esac
     else
-        opts=('id' 'label' 'name')
+        opts=('id' 'label' 'mode' 'name')
         _describe -t filter-opts "filter options" opts -qS "=" && ret=0
     fi