doc: update gb28181

Author: langhuihui

docs/protocols/gb28181.md

@@ -145,7 +145,8 @@ gb28181:
           - udp::5060    #SIP通讯端口,必填
   onsub:
     pull:
-      ^gb_\d+/(.+)$: $1    #播放GB设备请求URL的正则,例如deviceid,channelid分别为34020000001110000003,34020000001320000003,则在系统内的地址为http://localhost:8080/flv/gb_1/34020000001110000003/34020000001320000003.flv,其他协议类同
+      ^\d{20}/\d{20}$: $0  #播放GB设备请求URL的正则,例如deviceid,channelid分别为34020000001110000003,34020000001320000003,则在系统内的地址为http://localhost:8080/flv/34020000001110000003/34020000001320000003.flv,其他协议类同
+      ^gb_\d+/(.+)$: $1   #用来播放回放流，可以保证每个回放流进度不同
 ```
 
 ![GB28181 设备配置](/gb28181.png)