message: update example for register with dedicated codesandbox
diff: @@ -2,11 +2,17 @@ export default `import * as React from "react";
  import { useForm } from "react-hook-form";
  export default function App() {
  - const { register, handleSubmit } = useForm();
  - const onSubmit = (data) => alert(JSON.stringify(data));
  + const { register, handleSubmit } = useForm({
  + defaultValues: {
  + firstName: '',
  + lastName: '',
  + checkbox: [],
  + radio: ''
  + }
  + });
  return (
  - <form onSubmit={handleSubmit(onSubmit)}>
  + <form onSubmit={handleSubmit(console.log)}>
  <input {...register("firstName", { required: true })} placeholder="First name" />
  <input {...register("lastName", { minLength: 2 })} placeholder="Last name" />
  