docs: update webrtc example

[achingbrain]

Updates example to incorporate changes from #2776

This is done in a separate PR because doing it in #2776 would cause a major of @libp2p/webrtc to be released.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works