Readme: Update the code example for client.send_audio (#101)
Readme: Update the code example for client.send_audio (#101)

I tried the code as is, and I got 400 error:
```
Client error: 400 b'{"type":400,"title":"Bad Request","invalid_parameters":[{"reason":"Invalid type parameter provided","name":"stream_url"}]}'
```
According to the docs, stream_url accepts an array of strings.
Changing the stream_url to a list works.
Nexmo/nexmo-python