message: improve example code
diff: @@ -24,7 +24,7 @@ const Example = () => {
  <input
  name="username"
  ref={register({
  - validate: value => !(value === "admin") || "Nice try!"
  + validate: value => value !== "admin" || "Nice try!"
  })}
  />
  {errors.username && errors.username.message}
  