[webrtc-pc] stuns: usage in examples (#2270)

fippo has just created a new issue for https://github.com/w3c/webrtc-pc:

== stuns: usage in examples ==
The examples in this document show `stuns:` but AFAIK no browser actually supports stuns. webrtc.org has an open bug [here](https://bugs.chromium.org/p/webrtc/issues/detail?id=3644) and it hasn't seen much activity in the last years.

Might be easier to just say 'stun:'?

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2270 using your GitHub account