Fix equivalent sources citation

[leouieda]

Page that contains the error or needs to be improved (paste link):

Everywhere we talk about equivalent sources (docstrings and tutorials).

Description of the problem/suggestion:

We keep citing Cooper (2000) for the equivalent sources and I have no idea how this citation got there or why I never noticed it before. The correct citation is Cordell (1992).

I don't think we actually use the Cooper citation for anything in Harmonica so it would be a case of replacing it entirely with the proper citation to Cordell.