DOC: Update ASV benchmark example in contributing.rst
Updating the ASV example based on the cleaning that was done in `asv_bench/benchmarks/groupby.py`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19201?src=pr&el=h1) Report
> Merging [#19201](https://codecov.io/gh/pandas-dev/pandas/pull/19201?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/78c3ff97a8795c2de9861b6b0a40a5fa4b8e331f?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/19201/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19201?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19201      +/-   ##
==========================================
+ Coverage   91.51%   91.52%   +<.01%     
==========================================
  Files         147      147              
  Lines       48773    48773              
==========================================
+ Hits        44637    44639       +2     
+ Misses       4136     4134       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.89% <ø> (ø)` | :arrow_up: |
| #single | `41.62% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19201?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19201/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.41% <0%> (+0.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19201?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/19201?src=pr&el=footer). Last update [78c3ff9...a45dd2a](https://codecov.io/gh/pandas-dev/pandas/pull/19201?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!