DOC: fix RT03, ES01 for pandas.core.groupby.DataFrameGroupBy.agg and pandas.core.groupby.DataFrameGroupBy.aggregate
fixes

```
pandas.core.groupby.DataFrameGroupBy.agg RT03, ES01
pandas.core.groupby.DataFrameGroupBy.aggregate RT03, ES01
```
This is looking good to me, but we'll need to wait until the CI gets fixed in order to ensure the docs build correctly. Thanks @tuhinsharma121 