DOC: Fix flake8 issues in categorical.rst
- [x] closes #23793
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hi, @datapythonista , if you are free, could you take a look at the error message? i don't understand what caused this test to fail. thank you very much! Not sure what is wrong with that test, but looks unrelated, I think I've seen it failing in other PRs too. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23839?src=pr&el=h1) Report
> Merging [#23839](https://codecov.io/gh/pandas-dev/pandas/pull/23839?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/485e7cdfce8c98dba555a908579f8e10ce229430?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23839/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23839?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23839   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         161      161           
  Lines       51490    51490           
=======================================
  Hits        47521    47521           
  Misses       3969     3969
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.68% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23839?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23839?src=pr&el=footer). Last update [485e7cd...389fa49](https://codecov.io/gh/pandas-dev/pandas/pull/23839?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks a lot for these fixes @charlesdong1991 