DOC: work around IPython directive bug
Closes https://github.com/pandas-dev/pandas/issues/23954
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23994?src=pr&el=h1) Report
> Merging [#23994](https://codecov.io/gh/pandas-dev/pandas/pull/23994?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8ed347a25c34676399a46a4274c3dcf366f37ecf?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/23994/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23994?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23994   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         161      161           
  Lines       51538    51538           
=======================================
  Hits        47572    47572           
  Misses       3966     3966
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.38% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23994?src=pr&el=footer). Last update [8ed347a...0cabc16](https://codecov.io/gh/pandas-dev/pandas/pull/23994?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Fixed that issue in the doc build.