Inconsistent flag name in error message

[DissectBot]

in the utils.py, there are two instances where --output_file is used instead of --output-file this needs to be changed for consistency

https://github.com/fox-it/acquire/blob/8a3a0b5eaf3d6e251aa52b5cad7e0b49a22cf7cd/acquire/utils.py#L302