onSubmit type signature - is callback really optional?

### Are you submitting a **bug report** or a **feature request**?

bug report

### What is the current behavior?

https://final-form.org/docs/react-final-form/types/FormProps#onsubmit

```
callback?: (errors?: SubmissionErrors) => void,
```

### What is the expected behavior?

```
callback: (errors?: SubmissionErrors) => void,
```

I believe `callback` parameter is passed everytime, it's just matter of function itself if it wants to use it.