mute video in scroll demo

examples/scroll.html

@@ -95,7 +95,7 @@
             <h2>Image Backgrounds</h2>
             <pre><code class="hljs html">&lt;section data-background="image.png"&gt;</code></pre>
           </section>
-          <section data-background-video="https://s3.amazonaws.com/static.slid.es/site/homepage/v1/homepage-video-editor.mp4,https://s3.amazonaws.com/static.slid.es/site/homepage/v1/homepage-video-editor.webm">
+          <section data-background-video-muted data-background-video="https://s3.amazonaws.com/static.slid.es/site/homepage/v1/homepage-video-editor.mp4,https://s3.amazonaws.com/static.slid.es/site/homepage/v1/homepage-video-editor.webm">
             <h2>Video background</h2>
           </section>
         </section>