rustdoc_to_markdown: Clean up heading spacing (#12456)

This PR cleans up the spacing around the Markdown headings in the output
so that they are consistent.

Release Notes:

- N/A
abec028e58Marshall Bowers committed on 5/29/2024, 10:39:51 PM· committed by GitHubparent0888182
3 files changedLine totals unavailable