.unwrap() method has incorrect / out of date return type in documentation

[mknowlton89]

In the README, the documentation states the unwrap function will return a typed Payload, but looking at the source code, the unwrap function returns a generic Object type.

It would be ideal if this Payload type was defined instead of the generic Object as it will improve type safety and developer experience.

I do see there is an open PR that adds a comprehensive WebhookEvent type here that appears to resolve this issue.