How do I change samle_rate in output ogg file?

```py
import av

inp = av.open('f.mp3', 'r')
out = av.open('f.ogg', 'w')

ostream = out.add_stream('libopus')

for frame in inp.decode(audio=0):
    for packet in ostream.encode(frame):
        out.mux(packet)

out.close()
```