Add zsh completion for 'docker node rm --force'

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

contrib/completion/zsh/_docker

@@ -804,6 +804,7 @@ __docker_node_subcommand() {
         (rm|remove)
              _arguments $(__docker_arguments) \
                 $opts_help \
+                "($help)--force[Force remove an active node]" \
                 "($help -)*:node:__docker_complete_pending_nodes" && ret=0
             ;;
         (demote)