cmd/present: fix unclosed section element

Change-Id: I974ec7a2caccfca3aff18836d6495597e17664e7
Reviewed-on: https://go-review.googlesource.com/2533
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/present/templates/slides.tmpl

@@ -46,6 +46,8 @@
         {{end}}
       </article>
 
+    </section>
+
   </body>
   {{if .PlayEnabled}}
   <script src='/play.js'></script>