Update src/stream/stream/flatten.rs

Co-Authored-By: Taiki Endo <te316e89@gmail.com>

Author: k-nasa

src/stream/stream/flatten.rs

@@ -109,7 +109,7 @@ where
  let mut this = self.project();
  loop {
  if let Some(inner) = this.frontiter.as_mut().as_pin_mut() {
- if let item @ Some(_) = futures_core::ready!(Pin::new(inner).poll_next(cx)) {
+ if let item @ Some(_) = futures_core::ready!(inner.poll_next(cx)) {
  return Poll::Ready(item);
  }
  }