Fix xmpp URI or test chat in README.md

[erebion]

This fixes the URI of the example/test chat so that users click it to join.

[selfhoster1312]

Hello! Nice catch, but the correct URI for joining a group chat is xmpp:ADDRESS?join :-)

It's already fixed in our community fork README: https://github.com/matterbridge-org/matterbridge

[erebion]

Hello! Nice catch, but the correct URI for joining a group chat is xmpp:ADDRESS?join :-)

Are you sure the ?join is really required? I actually tested it. :D

[selfhoster1312]

I may be wrong, but i don't think it's defined what the client should do without ?join. The behavior with ?join is specified in XEP-0045 MUC since at least 20 years :)