message: make cfg(miri) greppable
diff: @@ -94,9 +94,9 @@ Linux program, you can do `cargo miri run --target x86_64-unknown-linux-gnu`.
  This is particularly useful if you are using Windows, as the Linux target is
  much better supported than Windows targets.
  -When compiling code via `cargo miri`, the `miri` config flag is set. You can
  -use this to ignore test cases that fail under Miri because they do things Miri
  -does not support:
  +When compiling code via `cargo miri`, the `cfg(miri)` config flag is set. You
  +can use this to ignore test cases that fail under Miri because they do things
  +Miri does not support:
  ```rust
  #[test]
  