DOC: added example of valid input dict in dfgroupby.aggregate
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

This edit adds an example in [dfgroupby.aggregate](https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.aggregate.html), for the more complicated case where the input is a "dict of axis labels -> functions, function names or list of such". It also explicitly states that it is possible to use `list` as a "function" (where the output will be a list of values for each group in the target column, as seen below).

![image](https://user-images.githubusercontent.com/90637415/173214630-6bdc6072-3d00-496c-8d52-49390f7d7f4f.png)
Hello @jasonjiajs! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-06-13 22:48:39 UTC Hi, this is my first attempt at contributing to the documentation. I've made what I think is a very small change, but it failed 2 tests and I'm not sure why. Would appreciate any help on this! Great, thanks @jasonjiajs 