fix type of set-box!

Author: AlexKnauth

typed-racket-lib/typed-racket/base-env/base-env.rkt

@@ -870,7 +870,7 @@
 [unbox (-poly (a) (cl->*
                    ((-box -Bottom a) . -> . a)
                    ((make-BoxTop) . -> . Univ)))]
-[set-box! (-poly (a) ((-box a) a . -> . -Void))]
+[set-box! (-poly (a) ((-box a -Bottom) a . -> . -Void))]
 [unsafe-unbox (-poly (a) (cl->*
                           ((-box a) . -> . a)
                           ((make-BoxTop) . -> . Univ)))]