Update typescript definition for headless render

[9oelM]

Thank you for writing this wonderful library. Really making use of it.

Just got one little thing: As per #259, receiving instance as a parameter in the callback method in render prop has become possible, but it seems that the example in README and typedef in index.d.ts have not yet been updated. Could we get them updated?

[KubaJastrz]

Certainly, do you want to do a PR?

[9oelM]

sure. will do soon

[atomiks]

Updated it