paper-radio-button demo should reference PaperInkyFocusBehavior instead of PaperInkyFocusBehaviorImpl

[bicknellr]

See https://github.com/PolymerElements/paper-behaviors/blob/7937ad3/demo/paper-radio-button.html#L112.

PaperInkyFocusBehaviorImpl should only be referenced internally by PaperInkyFocusBehavior.

[keanulee]

PaperInkyFocusBehavior is actually an array of behaviors, so there's no clean way of grabbing a reference to the right _createRipple. Also, since this is within a method called _createRipple, it's not possible to use the behavior and call it. Without proper inheritance, there's no way of doing this cleanly.