message: fix dead link in README
diff: @@ -156,7 +156,7 @@ Try deleting `~/.cache/miri`.
  This means the sysroot you are using was not compiled with Miri in mind. This
  should never happen when you use `cargo miri` because that takes care of setting
  up the sysroot. If you are using `miri` (the Miri driver) directly, see
  -[below][testing-miri] for how to set up the sysroot.
  +[CONTRIBUTING.md](CONTRIBUTING.md) for how to use `./miri`.
  ## Miri `-Z` flags and environment variables
  