fix(timeline): fixed htmlContents example (#2651)

examples/timeline/items/htmlContents.html

@@ -48,7 +48,7 @@
   item5.appendChild(document.createTextNode('item 5'));
   item5.appendChild(document.createElement('br'));
   var img5 = document.createElement('img');
-  img5.src = 'img/attachment-icon.png';
+  img5.src = '../resources/img/attachment-icon.png';
   img5.style.width = '48px';
   img5.style.height = '48px';
   item5.appendChild(img5);
@@ -72,4 +72,4 @@
   var timeline = new vis.Timeline(container, items, options);
 </script>
 </body>
-</html>
+</html>