Update the default AC-3 audio and pass-through bitrates in the `--help` output of `transcode-video` to 640 Kbps, matching the behavior of the code since version 0.5.0.
Update the default AC-3 audio and pass-through bitrates
in the `--help` output of `transcode-video` to 640 Kbps,
matching the behavior of the code since version 0.5.0.

JMoVS/video_transcoding,tonirogel/video_transcoding,donmelton/video_transcoding