DOC: fixed a minor mistake worksheet -> workbook
A small typo related to #38990
A worksheet can't have multiple worksheets, but a workbook can. Minor fix but may confuse the newcomers

- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

thanks @pilyavetsb 