Spring Use HttpStatusCode consistently in reference guide

Updated the first argument to onStatus(...) to use HttpStatusCode instead of HttpStatus.

Comment From: snicoll

Thanks for the PR. Both are valid but I can see how we want the doc to be consistent there.

Comment From: ypyf

Thanks for the PR. Both are valid but I can see how we want the doc to be consistent there.

If I understand correctly, Predicate<HttpStatus> could not safely assign to Predicate<HttpStatusCode> although HttpStatus is a subtype of HttpStatusCode.

Comment From: snicoll

You're right, I missed the link with the predicate.