message: Fix incorrect formatting in README.
diff: @@ -119,7 +119,7 @@ If your issues or pull requests have no response after a few days, feel free to
  For more details, see the [CONTRIBUTING.md file](https://github.com/japaric/criterion.rs/blob/master/CONTRIBUTING.md)
  -## Compatibility Policy
  +### Compatibility Policy
  Criterion.<span></span>rs supports the last three stable minor releases of Rust. At time of
  writing, this means Rust 1.25 or later. Older versions may work, but are not tested or guaranteed.
  