[1.x] update videocall plugin docs to remove/replace apprtc link

[sandeshkhadka]

What version of Janus is this happening on? 1.3.0

Have you tested a more recent version of Janus too? Yes

Was this working before? Yes

Is there a gdb or libasan trace of the issue? N/A (https://janus.conf.meetecho.com/docs/debug).

Additional context I was reading the janus docs and found the apprtc link mentioned in the videocall plugin (https://apprtc.appspot.com) returns 404 error. After visiting apprtc's github I got to know it has been shutdown. It might be good to update the docs to either replace the link with apprtc's github repo(https://github.com/webrtc/apprtc) or simply remove the link. I can raise a PR if you wish

[lminiero]

Thanks for the heads up! No need for a PR, I'll fix it myself.