docs: link to working example for "try online" (#9244)

It's confusing that people can an immediate error when they click the
link.

Thanks @mkery for the suggestion.

site/_layouts/plain.html

@@ -23,7 +23,7 @@
         </div>
         <div class="secondary-links">
           <a href="https://www.github.com/vega/vega-lite">GitHub</a>
-          <a href="https://vega.github.io/editor/#/custom/vega-lite">Try Online</a>
+          <a href="https://vega.github.io/editor/#/examples/vega-lite/bar">Try Online</a>
         </div>
       </div>
     </nav>