I modified an already modified script under GPL v3 and want to allow optional academic citations in my package

A section for "how to cite" is legally speaking a nice-to-have part often found in repositories used for academic research. It is not a requirement by the licenses which require that proper attribution is given or retained within the repository as to not mis-represent who contributed and what is the license.

The suggestion on how to cite generally should be treated with similar measure like author information for license considerations though. While you have the right to remove the suggestion and replace it exclusively by your own, doing so might not be the wisest choice - but depends on how much you modified the content of the repository and the code's functionality.

The likely best approach is to amend the quote suggestion such that you either add an additional line which refers to your repository or to modify the existing in a way that both you and the original authors get credit (but possibly only linking to your repo).

A suggestion following the suggested quotation from the link you provided could be

Name, Your (2020); Pereira, R.H.M.; Gonçalves, C.N.; et. al (2019) geobr: Loads Shapefiles of Official Spatial Data Sets of Brazil. GitHub repository - https://example.com/YourRepositoryLink.