关于emqx_sn的操作文档

File: [/zh_CN/modules/mqtt_sn_protocol.md](https://docs.emqx.cn/enterprise/v4.3/modules/mqtt_sn_protocol.html)

文档中关于dashboard的操作指引与实际dashboard的界面不符，无法进行复现，希望能够及时更新

Start with zh_CN/modules/mqtt_sn_protocol.md and compare its Dashboard operation steps with the current Dashboard interface. Update the instructions so the MQTT-SN procedure can be reproduced, then verify the revised documentation against the actual interface.

Written by the indexing model from the issue text.