isbn-verifier: Use second-edition links in README
isbn-verifier: Use second-edition links in README

We forgot to apply https://github.com/exercism/rust/pull/368 to
https://github.com/exercism/rust/pull/378

Further, it seems this was forgotten in
https://github.com/exercism/rust/pull/382

sacherjj/rust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,sacherjj/rust,sacherjj/rust,petertseng/exercism-rust