message: fix CONTRIBUTING example
diff: @@ -74,7 +74,7 @@ You can get a trace of which MIR statements are being executed by setting the
  `MIRI_LOG` environment variable. For example:
  ```sh
  -MIRI_LOG=info ./miri run tests/run-pass/vecs.rs
  +MIRI_LOG=info ./miri run tests/run-pass/vec.rs
  ```
  Setting `MIRI_LOG` like this will configure logging for Miri itself as well as
  @@ -83,7 +83,7 @@ can also do more targeted configuration, e.g. the following helps debug the
  stacked borrows implementation:
  ```sh
  -MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/run-pass/vecs.rs
  +MIRI_LOG=rustc_mir::interpret=info,miri::stacked_borrows ./miri run tests/run-pass/vec.rs
  ```
  In addition, you can set `MIRI_BACKTRACE=1` to get a backtrace of where an
  