DOC: Fix PEP-8 issues in groupby.rst
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23900?src=pr&el=h1) Report
> Merging [#23900](https://codecov.io/gh/pandas-dev/pandas/pull/23900?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a7b187ab42c48342478ed8664145f21578f50622?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/23900/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23900?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23900   +/-   ##
=======================================
  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: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23900?src=pr&el=footer). Last update [a7b187a...bbf6eaf](https://codecov.io/gh/pandas-dev/pandas/pull/23900?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @FHaase !  Keep them coming!