Updated css in example 12

examples/timeline/12_custom_styling.html

@@ -7,7 +7,7 @@
   <link href="../../dist/vis.css" rel="stylesheet" type="text/css" />
 
   <style type="text/css">
-    .vis.timeline.rootpanel {
+    .vis.timeline.root {
       border: 2px solid purple;
       font-family:  purisa, 'comic sans', cursive;
       font-size: 12pt;