DOC: clean doc string for NDFrame.astype
Using astype(ordered..., categories=...) has been deprecated. This PR cleans a usage of this pattern in a doc string.

Hello @topper-123! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/topper-123/pandas/blob/3dc5b0d2cdcf5f03b05b2d1a9efca4475aceca97/pandas/core/generic.py) !

 thanks @topper-123 