Updating link to contribution guidelines

index.md

@@ -14,4 +14,4 @@ please see [the setup instructions]({{ site.github.url }}/setup/).
 > Use the `.prereq` style to specify prerequisites.
 {: .prereq}
 
-[contributing]: https://github.com/{{ site.account }}{{ site.root }}/blob/gh-pages/CONTRIBUTING.md
+[contributing]: {{ site.github.repository_url }}/blob/gh-pages/CONTRIBUTING.md