Fix puget printer future handler

[onbreath]

Hi, the puget printer was trying to check future-done? on clojure.core/promise.
Couldn't run the tests locally, can do that later too.

[onbreath]

Sorry for the short initial message, I was in a hurry.
For context, future-done? on clojure.core/promise is really checking the core var. This seems to have been left over in a refactor. The resulting error is just as confusing as my initial PR-message.
I'm using deep-diff2 to diff internal representations of Clay notebooks between two algorithms to parse/read and eval Clojure code. Since this is going to run over all of the Clojure Civitas posts amongst other things, it's exercising quite some code paths in deep-diff2.