Fix variable in the docs

[JakubAndrysek]

Incorrect variable calling

https://github.com/diegomura/react-pdf-site/blob/master/docs/on-the-fly.md?plain=1#L96

Please change

  if (instance.error) return <div>Something went wrong: {error}</div>;

To

  if (instance.error) return <div>Something went wrong: {instance.error}</div>;