Update tests/pass-dep/tokio/issue-3858.rs

Co-authored-by: Ralf Jung <post@ralfj.de>

tests/pass-dep/tokio/issue-3858.rs

@@ -27,7 +27,7 @@ fn main() {
  let newfd = unsafe { libc::dup(epfd) };
  assert_ne!(newfd, -1);
 
- // Close the epoll instance.
+ // Close the old epoll instance, so the new FD is now the only FD.
  let res = unsafe { libc::close(epfd) };
  assert_eq!(res, 0);