example: add GitHub markdown CSS (#434)

Makes the output a lot prettier
diff --git a/example/index.html b/example/index.html
index ba9d207..989b557 100644
--- a/example/index.html
+++ b/example/index.html
@@ -5,6 +5,7 @@
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Roboto+Mono|Roboto">
     <link rel="stylesheet" href="https://fonts.googleapis.com/icon?family=Material+Icons+Extended">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.0.1/styles/default.min.css">
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/github-markdown-css/5.1.0/github-markdown-light.min.css" integrity="sha512-zb2pp+R+czM7GAemdSUQt6jFmr3qCo6ikvBgVU6F5GvwEDR0C2sefFiPEJ9QUpmAKdD5EqDUdNRtbOYnbF/eyQ==" crossorigin="anonymous" referrerpolicy="no-referrer" />
     <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.0.1/highlight.min.js"></script>
     <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.0.1/languages/dart.min.js"></script>
     <script defer src="app.dart.js"></script>
@@ -37,7 +38,7 @@
           <div class="toolbar">
             <h2>HTML</h2>
           </div>
-          <div id="html"></div>
+          <div id="html" class="markdown-body"></div>
         </div>
       </div>
       <footer>