Update src/stream/stream/flatten.rs

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

src/stream/stream/flatten.rs

@@ -114,7 +114,7 @@ where
                 }
             }
 
-            match futures_core::ready!(Pin::new(&mut this.stream).poll_next(cx)) {
+            match futures_core::ready!(this.stream.as_mut().poll_next(cx)) {
                 None => return Poll::Ready(None),
                 Some(inner) => *this.frontiter = Some(inner.into_stream()),
             }