CodeSandbox invalid for simple example in documentation

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

Bug

### What is the current behavior?

It shows a "not found" message for the example.

### What is the expected behavior?

It should show the codesandbox.

### Sandbox Link

N/A

### What's your environment?

N/A

### Other information

Link to the page in the documentation https://final-form.org/docs/react-final-form/examples/simple

Screenshot of the issue