Update archive-single.html

In the "archive-single" include, the "Download" word is repeated twice in the case when only a slidesURL is given. 
An extra paragraph ending is also mistakenly present.

Author: opasche

_includes/archive-single.html

@@ -63,7 +63,7 @@ <h2 class="archive__item-title" itemprop="headline">
     {% elsif post.paperurl %}
       <p><a href=" {{ post.paperurl }} ">Download Paper</a></p>
     {% elsif post.slidesurl %}
-      <p>Download <a href="{{ post.slidesurl }}">Download Slides</a></p></p>
+      <p><a href="{{ post.slidesurl }}">Download Slides</a></p>
     {% endif %}
 
   </article>