Update index.html

Author: coding-horror

index.html

@@ -130,7 +130,7 @@ <h3>
                     Quick reference card and interactive tutorial for learning Markdown.
                 </p>
                 <h3>
-                    <a href="https://spec.commonmark.org/dingus/">spec.commonmark.org/dingus/</a>
+                    <a href="https://spec.commonmark.org/dingus/">spec.commonmark.org/dingus</a>
                 </h3>
                 <p>
                     Live testing tool powered by the reference implementation.