UpdateCitation: use update_citations instead of insert_citations

[Yakimych]

Hi. Was looking to use the UpdateCitation mutation in the flutter app and noticed that insert_citations with on_conflict is being used here. Is there any reason not to use update_citations instead of insert_citations for this?

https://github.com/MargaretKrutikova/re-cite/blob/master/src/core/Mutations.re#L38