Run Resyntax on markdown renderer

[jackfirth]

This pull request uses Resyntax to clean up the scribble/markdown-render module. It's using a newer version of Resyntax which also uses fmt to format the fixed code. I'm opening this PR to get feedback on whether the formatting changes seem like an improvement.

[racket-discourse-github-bot]

This pull request has been mentioned on Racket Discourse. There might be relevant details there:

https://racket.discourse.group/t/resyntax-now-uses-fmt-to-format-suggestions/3120/2

[mflatt]

Thanks! Should I squash these commits to merge, to keep them separate?

[jackfirth]

I recommend squashing. They're only separate because Resyntax doesn't do multiple passes at once yet.