Readme incorrect for environment variable names.


Hi

The README is incorrect regarding the naming of environment variables.

>BAR_BEAM_RATIO
>The share of (available) horizontal display real estate the progress bar beam should occupy (default: 0.75)
>Example
```sh
$> BAR_BAR_SIZE_RATIO=0.5 ffmpeg-bar -i in.mp4 output.mp4
```

This should in fact be "BAR_SIZE_RATIO".

