update apachecon links to https (#12521)

* update apachecon links to https

* nudging file to get past flakey test

docs/_static/mxnet-theme/index.html

@@ -15,7 +15,7 @@
           </div>
       </div>
       <div class="col-lg-6" style="padding-top: 60px;">
-        <a href="http://www.apachecon.com/acna18/" class="section-tout-promo"><img src="http://www.apachecon.com/acna18/banners/acna-sleek-highres.png" width="65%" alt="apachecon"/></a>
+        <a href="https://www.apachecon.com/acna18/" class="section-tout-promo"><img src="https://www.apachecon.com/acna18/banners/acna-sleek-highres.png" width="65%" alt="apachecon"/></a>
       </div>
     </div>
   </div>
@@ -42,7 +42,6 @@ <h3>Introducing the Scala Inference API</h3>
     </div>
   </div>
 </div>
-
 <div class="section-util">
     <div class="container">
       <div class="row">