DOC/TST: Fix warning in concat docstring
xref https://github.com/pandas-dev/pandas/issues/23149
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/reshape/concat.py`](https://github.com/TomAugspurger/pandas/blob/dc9fadf1e7234dca4e6760da82f7ef3934303692/pandas/core/reshape/concat.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23529?src=pr&el=h1) Report
> Merging [#23529](https://codecov.io/gh/pandas-dev/pandas/pull/23529?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bd98841b2bc0998565981b793e012c0adb1e8983?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23529/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23529?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23529      +/-   ##
==========================================
+ Coverage   92.25%   92.25%   +<.01%     
==========================================
  Files         161      161              
  Lines       51181    51186       +5     
==========================================
+ Hits        47217    47222       +5     
  Misses       3964     3964
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23529?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/23529/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.6% <ø> (ø)` | :arrow_up: |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/23529/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `96.51% <0%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23529?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/23529?src=pr&el=footer). Last update [bd98841...dc9fadf](https://codecov.io/gh/pandas-dev/pandas/pull/23529?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
