improve top example in tutorial

examples/tutorial.html

@@ -60,12 +60,16 @@
       <p>We can also cite <dt-cite key="gregor2015draw"></dt-cite> external publications.</p>
     </dt-article>
 
+    <dt-appendix>
+    </dt-appendix>
+
     <script type="text/bibliography">
       @article{gregor2015draw,
         title={DRAW: A recurrent neural network for image generation},
         author={Gregor, Karol and Danihelka, Ivo and Graves, Alex and Rezende, Danilo Jimenez and Wierstra, Daan},
         journal={arXivreprint arXiv:1502.04623},
-        year={2015}
+        year={2015},
+        url={https://arxiv.org/pdf/1502.04623.pdf}
       }
     </script>
   </dt-code>