docs: improve pure js example

Author: elevatebart

demo/assets/input.js

@@ -1,8 +1,13 @@
 new Vue({
-  template: '<input type="checkbox" :name="cname">',
+  template: `
+  <div style="padding:30px">
+  <input v-model="value" type="checkbox" :name="cname">
+  <h1 v-if="value">I am checked</h1>
+  </div>`,
   data() {
     return {
-      cname: "myCheck"
+      cname: "myCheck",
+      value: false
     };
   }
 });