Docs: call superclass constructors in radio.sky example

Review URL: https://codereview.chromium.org/689763003

examples/radio.sky

@@ -13,6 +13,7 @@ SKY MODULE - radio button and radio button group
  module.exports = {};
  module.exports.RadioElement = sky.registerElement('radio', class extends Element {
    constructor () {
+     super();
      this.addEventListener('click', (event) => this.checked = true);
      this.addShadowRoot(new sky.ShadowRoot(module.document.findId('radio-shadow').content.cloneNode(true)));
    }
@@ -48,6 +49,7 @@ SKY MODULE - radio button and radio button group
 <script>
  module.exports.RadioGroupElement = sky.registerElement('radiogroup', class extends Element {
    constructor () {
+     super();
      this.addShadowRoot(new sky.ShadowRoot(module.document.findId('radiogroup-shadow').content.cloneNode(true)));
    }
    get value () {