update link to config.py in options user guide
- `config.py` now `config_core.py` ?!
- maybe also [pandas/_config/config.py](https://github.com/pandas-dev/pandas/blob/master/pandas/_config/config.py)

- link in [`user_guide/options` ](https://pandas.pydata.org/pandas-docs/stable/user_guide/options.html) changed. 