sync: change oneshot poll_close to poll_closed

The action of `Sender::poll_close` is to check if the receiver has been
closed, not to try to close the sender itself. So change to
`poll_closed`.

tokio-sync/tests/fuzz_oneshot.rs

@@ -90,7 +90,7 @@ impl<'a> Future for OnClose<'a> {
     type Output = ();
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
-        self.get_mut().tx.poll_close(cx)
+        self.get_mut().tx.poll_closed(cx)
     }
 }