Update torproject.org and git.torproject.org 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://expyuzz4wqqyqhjn.onion/ -> http://2gzyxa5ihm7nsggfxnu52rck2vv4rvmdlkiu3zzui5du4xyclen53wid.onion/


http://dccbbv6cooddgcrq.onion/ -> http://xtlfhaspqtkeeqxk6umggfbr3gyfznvf4jhrge2fujz53433i2fcs3id.onion/