fix broken youtube URL on website (trivial)

https://www.torproject.org/getinvolved/volunteer.html.en

links to https://www.youtube.com/c/thetorproject

which says: This channel does not exist.

fix: https://github.com/nusenu/torproject-webwml/commit/498ff4bb436df033145ab02d55837e8b163ed56d

branch: https://github.com/nusenu/torproject-webwml/tree/fix_youtube_url