doc: update systemctl.md (#578)

Author: loprx

command/systemctl.md

@@ -23,7 +23,9 @@ systemctl
 ```shell
 systemctl start nfs-server.service . # 启动nfs服务
 systemctl enable nfs-server.service # 设置开机自启动
+systemctl enable nfs-server.service --now # 设置开机自启动，并立刻启动
 systemctl disable nfs-server.service # 停止开机自启动
+systemctl disable nfs-server.service --now # 停止开机自启动，并立刻停止
 systemctl status nfs-server.service # 查看服务当前状态
 systemctl restart nfs-server.service # 重新启动某服务
 systemctl list-units --type=service # 查看所有已启动的服务