Fix monospace font fallback

`monospace` is more correct that `sans-serif` for "GeistMono". It
renders better for mobile readers who adblock all fonts. This doesn't
affect normal readers.

Author: danielzgtg

tailwind.config.js

@@ -24,7 +24,7 @@ module.exports = {
       fontFamily: {
         header: ['"Geist"', "sans-serif"],
         text: ['"Geist"', "sans-serif"],
-        mono: ['"GeistMono"', "sans-serif"],
+        mono: ['"GeistMono"', "monospace"],
       },
       borderWidth: {
         thin: "max(1px, 0.0625rem)",