The new `ruff rule` output format introduced in
551b810aeb doesn't print Markdown but
rather some rich text with escape sequences for colors and links,
it's actually the "text" format that prints Markdown, so naming the new
format "markdown" is very confusing. This commit therefore renames it to
"pretty".
This isn't a breaking change since there hasn't been a release yet.
Charlie Marsh
Martin Fischer
Nick Pope
Micha Reiser