Preformatted elements fallback to UA monospace

Courier looks mighty ugly on a lot of systems. When in doubt, it’s
better to just trust the user agent. This also allow users to set their
own preferred option—which is better than a bad option.

Author: toastal

src/html_support_files/odoc.css

@@ -500,7 +500,7 @@ h4 {
 /* Preformatted and code */
 
 tt, code, pre {
-  font-family: "Fira Mono", courier;
+  font-family: "Fira Mono", monospace;
   font-weight: 400;
 }