DOC: Remove use of as_type for PeriodIndex to DatetimeIndex conversion
- [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

Followup to #44398. Should get docs build to green. cc @jbrockmendel 

![image](https://user-images.githubusercontent.com/45562402/141684919-f8e79f49-a2c8-4001-bd2d-13849e5c0b41.png)

