Corrected 'columns' argument of 'to_csv' method
Compared the 'to_csv' method described in https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.to_csv.html with the same method described in https://pandas.pydata.org/pandas-docs/stable/io.html#writing-to-csv-format and noticed difference between the two in the 'columns' and 'cols' argument.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks @mobesa 