Fix outdated comment on Box

Author: bjorn3

library/alloc/src/boxed.rs

@@ -191,8 +191,7 @@ mod thin;
 #[fundamental]
 #[stable(feature = "rust1", since = "1.0.0")]
 // The declaration of the `Box` struct must be kept in sync with the
-// `alloc::alloc::box_free` function or ICEs will happen. See the comment
-// on `box_free` for more details.
+// compiler or ICEs will happen.
 pub struct Box<
     T: ?Sized,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,