Improve useFormState docs (#863)

useFormState returns a FormState and not a FormSpyRenderProps.

Author: bostrom

docs/api/useFormState.md

@@ -6,6 +6,6 @@
 import { useFormState } from 'react-final-form'
 ```
 
-The `useFormState()` hook takes one optional parameter, which matches the exact shape of [`FormSpyProps`](../types/FormSpyProps) (except without the render props). It returns a [`FormSpyRenderProps`](../types/FormSpyRenderProps).
+The `useFormState()` hook takes one optional parameter, which matches the exact shape of [`FormSpyProps`](../types/FormSpyProps) (except without the render props). It returns a [`FormState`](/docs/final-form/types/FormState).
 
 `useFormState()` is used internally inside [`<FormSpy/>`](FormSpy).