Update Support portal onion service link to v3 in README.md

As v2 onions are being removed, we need to update the site onion service in README.md file:

http://4bflp2c4tnynnbes.onion/

New onion: http://rzuwtpc4wb3xdzrj3yeajsvm3fkq4vbeubm2tdxaqruzzzgs5dwemlad.onion/