DOC: remove is_lexsorted from MultiIndex docstring
`is_lexsorted` has been deprecated (https://github.com/pandas-dev/pandas/pull/38701)

- [x] closes https://github.com/pandas-dev/pandas/issues/38953
- [ ] tests added / passed
- [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
- [ ] whatsnew entry
