DOC: fix a warning using obsolete package
currently there is a warning:
"/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/IPython/html.py:12: ShimWarning: The `IPython.html` package has been deprecated since IPython 4.0. You should import from `notebook` instead. `IPython.html.widgets` has moved to `ipywidgets`.
  warn("The `IPython.html` package has been deprecated since IPython 4.0. ""

this change fixes the warning.

- [ ] closes #xxxx
- [ ] 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
