[PeerTube] Fix parsing id for instances whose domain ends with a or c

[TobiGr]

• [x] I carefully read the contribution guidelines and agree to them.
• [x] I have tested the API against NewPipe.
• [x] I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.

The pattern to detect the channel ID was faulty and e.g. the ID detected for https://kolektiva.media/video-channels/documentary_channel was a/video-channels which is wrong. A new pattern is added to distinguish between URLs and potential IDs because IDs must not start with / while IDs inside an URL must.

Fixes TeamNewPipe/NewPipe#11369