DOC: fix an incomplete sentence
"We will address array-based indexing like s[[4, 3, 1]] in section." -> "We will address array-based indexing like s[[4, 3, 1]] in section on indexing."

- [ ] 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
