DOC: Fix warnings on underline length
In the CI run
 https://github.com/pandas-dev/pandas/runs/5413504567?check_suite_focus=true
I noticed a few warnings when building the docs such as

```
/home/runner/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/numpydoc/docscrape.py:434: UserWarning: potentially wrong underline length...
Methods
---------- in
Immutable sequence used for indexing and alignment. The basic object
storing axis labels for all pandas objects. Int64Index is a special case... in the docstring of Int64Index in /home/runner/work/pandas/pandas/pandas/core/indexes/numeric.py.
```

So these were trivial enough to clean up

- [x] (simple enough I didn't make one) closes #xxxx (Replace xxxx with the Github issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

The ASV check failed because of https://github.com/pandas-dev/pandas/pull/46193, this change is innocent. thanks @NickCrews 