stickermule link update (#1129)

Author: sjmiller7

index.html

@@ -454,7 +454,7 @@ <h2>Partners</h2>
             <a href="http://mlh.io" class="logo mlh" target="_blank">
                 <img src="./assets/sponsors/logos/mlh-white.svg" />
             </a>
-            <a href="http://stickermule.com" class="logo stickermule" target="_blank">
+            <a href="http://hackp.ac/mlh-stickermule-hackathons" class="logo stickermule" target="_blank">
                 <img src="./assets/sponsors/logos/stickermule-white.svg" />
             </a>
         </div>