docs(example): Updated the paper-radio-group example with custom events.

@jbdeboer for my information, how do you get the event name ? (from the [source code](https://github.com/Polymer/paper-radio-group/blob/ad06ae0ccc10a5088dbcfb38062fe9dde65f11ba/paper-radio-group.html) I would say that event is `change` and I can not find anything usefull in the doc)