Add a URL for the hoedown link

Author: carols10cents

_posts/2018-01-04-Rust-1.23.md

@@ -42,6 +42,7 @@ October of last year], so many crates have already fixed any issues that
 they've found.
 
 [CommonMark]: http://commonmark.org/
+[Hoedown]: https://github.com/hoedown/hoedown
 [warn if there are any differences]: https://github.com/rust-lang/rust/pull/45324
 [written a blog post]: https://blog.guillaume-gomez.fr/articles/2017-09-18+New+rustdoc+rendering+common+errors
 [warning landed in nightly in May of last year]: https://github.com/rust-lang/rust/pull/41991