fix type signature of force for promises

force acts as the identity when given a non-promise; the previous type didn't allow it to accept non-promises.

for easy reference:
https://github.com/plt/racket/blob/master/racket/collects/racket/private/promise.rkt#L112

http://docs.racket-lang.org/reference/Delayed_Evaluation.html?q=force#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29