fix type for unbox

Author: AlexKnauth

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

@@ -868,7 +868,7 @@
 [box (-poly (a) (a . -> . (-box a)))]
 [box-immutable (-poly (a) (a . -> . (-Read-Only-Boxof a)))]
 [unbox (-poly (a) (cl->*
-                   ((-box a) . -> . a)
+                   ((-box -Bottom a) . -> . a)
                    ((make-BoxTop) . -> . Univ)))]
 [set-box! (-poly (a) ((-box a) a . -> . -Void))]
 [unsafe-unbox (-poly (a) (cl->*