DOC: Modify pandas.ExcelWriter default engine in docstring (#43359)
* engine for xlsx : xlsxwriter instead of openpyxl
* correction to pass the doctest

- [x] closes #43359
- [x] tests added / passed : script/validate_docstrings.py detected 3 errors before my pull request. It's now resolved
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them : black pandas didn't change anything
- [x] whatsnew entry : one word changed in docstring


Script validate_docstings report : [validate_docstrings.txt](https://github.com/pandas-dev/pandas/files/7117843/validate_docstrings.txt)

Thanks @LiquidMika 