Updates docs: fields => fields_csv and tables => tables_csv

[waylan]

The docs list the arguments on flatterer.flatten as fields and tables for "Python Usage." However, when used, they result in TypeError: flatten() got an unexpected keyword argument 'fields' and TypeError: flatten() got an unexpected keyword argument 'tables'. As can be seen at flatterer/__init__.py#73 and flatterer/__init__.py#75, the arguments are actually fields_csv and tables_csv respectively. This change updates the documentation to reflect the correct argument names.

Note that the arguments for flatterer.cli are fields and tables. But that function is not documented for Python usage.