message: use `cargo +nightly` rather than `rustup run nightly cargo`
diff: @@ -16,7 +16,7 @@ rustup update nightly
  You should also make `nightly` the default version for your Miri directory by
  running the following command while you're in it. If you don't do this, you can
  -run the later `cargo` commands by prefixing them with `rustup run nightly`.
  +run the later `cargo` commands by using `cargo +nightly` instead.
  ```sh
  rustup override add nightly
  