Update overview.md

[aaronjohnson]

Fixed a broken link to an external PDF.

Perhaps there is a more canonical location.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

coverage: 98.783%. remained the same
when pulling 202a597 on aaronjohnson:patch-1
into e45b999 on uber:master.

[ajfriend]

I think, technically, we should be pointing to the publisher's website here: https://www.tandfonline.com/doi/abs/10.1559/152304003100011090

Unfortunately, the PDF is paywalled at that link, but that's the proper way to handle this. But hopefully it isn't too hard for an enterprising person to find legitimate access to the PDF via other sites (including Dr. Sahr's website). I just don't think we can link it directly here.

[sahrk]

I think, technically, we should be pointing to the publisher's website

Agreed. The paper is owned by the publisher, so the only legal link to it is the publisher's website. I note that the paper is available on LibGen.