DOC: small typo fix
"functionaility" -> "functionality"

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19901?src=pr&el=h1) Report
> Merging [#19901](https://codecov.io/gh/pandas-dev/pandas/pull/19901?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92dbc78af9f34af83691d86347fc04d2c543b94c?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/19901/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19901?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19901   +/-   ##
=======================================
  Coverage   91.66%   91.66%           
=======================================
  Files         150      150           
  Lines       48938    48938           
=======================================
  Hits        44860    44860           
  Misses       4078     4078
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <ø> (ø)` | :arrow_up: |
| #single | `41.81% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19901?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/19901/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.31% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19901?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/19901?src=pr&el=footer). Last update [92dbc78...dd5804b](https://codecov.io/gh/pandas-dev/pandas/pull/19901?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!