Broken URLs in README.md

Hey, there are few broken links in the README.md. Everything that's pointing to https://tcdi.github.io/plrust
is 404. I'm not sure where these links should point to, or else I'd have created a PR as a fix.

It looks like they should be changed to s/tcdi/pgcentralfoundation/. That seemed to work on a random link I tried (and it redirected me to plrust.io). Don't have the bandwidth to check all the links and open a PR right now, but might do later if I remember.