message: Reformat the changelog
  Summary: Use headings properly, to make markdown lint not complain, and also because it looks better.
diff: -### Starlark
  +# Starlark
  -Unreleased
  +## Unreleased
  * Move to a garbage collector.
  * Move repo to https://github.com/facebookexperimental/starlark-rust.
  * Change maintainer to Facebook.
  -0.3.1 and before
  +## 0.3.1 and before
  -* The code was developed at https://github.com/google/starlark-rust
  +* The code was developed at https://github.com/google/starlark-rust.
  