Fix incorrect duration calculation in demo app.

Demo app used Date for convert total seconds to hours, minutes and seconds.
It set Date in UTC format but read in local time zone.
As result localtime zone hours count added to output.

src/demo/Duration.js

@@ -10,9 +10,9 @@ export default function Duration ({ className, seconds }) {
 
 function format (seconds) {
   const date = new Date(seconds * 1000)
-  const hh = date.getHours()
-  const mm = date.getMinutes()
-  const ss = pad(date.getSeconds())
+  const hh = date.getUTCHours()
+  const mm = date.getUTCMinutes()
+  const ss = pad(date.getUTCSeconds())
   if (hh) {
     return `${hh}:${pad(mm)}:${ss}`
   }