Fix format of advanced.rst according to PEP-8 standard
- [x] closes #23790 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23893?src=pr&el=h1) Report
> Merging [#23893](https://codecov.io/gh/pandas-dev/pandas/pull/23893?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/002ab434d6c9a6b6878da227814851ef0bc81301?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/23893/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23893?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23893   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         161      161           
  Lines       51498    51498           
=======================================
  Hits        47530    47530           
  Misses       3968     3968
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <ø> (ø)` | :arrow_up: |
| #single | `42.43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23893?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/23893/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23893?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23893?src=pr&el=footer). Last update [002ab43...5e724e7](https://codecov.io/gh/pandas-dev/pandas/pull/23893?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 cool, thanks @datapythonista  Thanks @YuechengWu ! @datapythonista : I presume it's safe now to turn on `flake8-rst` ? @gfyoung My pleasure! @gfyoung we have `flake8-rst` activated, but the version we have doesn't check `ipython` blocks. We're upgrading the version in #23847, but we still have plenty of files to fix. @datapythonista : Ah, right.  Thanks for clarifying!