DOC: remove grammar duplication in groupby docs
"can be could be" -> "could be"
Marking this for `0.24.0`, as this is a relatively trivial PR, and we can pick this up and bring to the finish line (I only have one comment, otherwise, LGTM) if it goes stale. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21534?src=pr&el=h1) Report
> Merging [#21534](https://codecov.io/gh/pandas-dev/pandas/pull/21534?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6131a598a614b93e70a28b36d35819cb6081965b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21534/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21534?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21534      +/-   ##
==========================================
- Coverage   91.92%   91.92%   -0.01%     
==========================================
  Files         153      153              
  Lines       49594    49594              
==========================================
- Hits        45590    45588       -2     
- Misses       4004     4006       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.82% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21534?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21534/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.75% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21534?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/21534?src=pr&el=footer). Last update [6131a59...e2ae4d1](https://codecov.io/gh/pandas-dev/pandas/pull/21534?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @adamjstewart Thanks!