DOC: Fix F821 error in docstring
- [ ] xref #22900 (I think this PR can also close the linked issue as now `pandas` and `numpy` are considered defaults import before linting docstring)

The command below returns no error now.

```
flake8 --doctest --ignore="" --select=F821 pandas/ | grep -v "F821 undefined name 'pd'" | grep -v "F821 undefined name 'np'"
```
@tqa236 should we add a check in the CI for this? Hello @datapythonista, I fully agree that fixing linting errors without enforcing them is quite inefficient. But I wonder if the CI check is a long term solution as it's based on `flake8 --doctest` while we're moving more and more to `ruff`.

There's an open issue on `ruff` to track this feature: https://github.com/astral-sh/ruff/issues/3542

In my opinion, we can wait for the feature to be implemented in `ruff` and enable it here rather than implementing our own check with `flake8 --doctest` for 2 reasons
- The code in all of the official docstrings are already run and checked if there's any syntax errors already.
- What's left for this CI check is only the internal docstring (in `conftest`,... ), and the number of errors is quite low (it in fact reduces compared to your result last year from the issue). 

Let me know what you think. Sounds good, up to you, I was just wondering if you considered it. Thanks for all the details and for the continued work on this. Thanks @tqa236 