Fix Audio docstring by removing unsupported mono argument
I think we can keep the arg and add the missing torch.mean() in the Audio.decode_example method > I think we can keep the arg and add the missing torch.mean() in the Audio.decode_example method

Thank you @lhoestq. I will add torch.mean(). fixed by #7840 
This PR fixes issue #7745.

Who can review:
@lhoestq 