DOC: Fix Series nlargest and nsmallest doc strings (#43964)
- [x] closes #43937
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] Cancel nlargest and nsmallest's doc list item text bold effect, detail could see below.

From
![Clip_20211011_094600](https://user-images.githubusercontent.com/25895405/136727253-30d20e21-ba1c-4be0-b1ad-87239b66d59e.png)
To
![Clip_20211011_105541](https://user-images.githubusercontent.com/25895405/136727262-f28a75e0-48e8-478b-a460-7690181d97d0.png)