Update example `Field Warnings`

**What is the current behavior?**

The 'Field Warnings' item listed in the examples section contains outdated versions of: react, react-dom, react-final-form and final-form. It is not working on newer versions of the final reaction form.

Using the `mutator` setting in `FormProps.render` the old way.

**What is the expected behavior?**

`Mutators` must be set in` FormProps.render` through the `form ?: FormApi` attribute.

**Sandbox Link**
Link described in the example section of `react-final-form`.
[Old example link](https://codesandbox.io/s/m5qwxpr6o8)

I forked the example of `react-final-form`, updated the dependencies and added the correct setting.
[Updated example link](https://codesandbox.io/s/react-final-form-warnings-5qt3v)

**Other information**
I took the initiative to create a fork and update the example and posted the link as a reference, but the team can decide to use my fork or update the existing version.

Thank you!