preface (Datasets not Data Sets)

[hardin47]

Throughout the book, we said dataset (instead of data set). The preface cites the openintro package:

Mine Çetinkaya-Rundel, David Diez, Andrew Bray, Albert Kim, Ben Baumer, Chester Ismay and Christopher Barr (2020). openintro: Data Sets and Supplemental Functions from 'OpenIntro' Textbooks and Labs. R package version 2.0.0. https://github.com/OpenIntroStat/openintro.

When/if the package gets changed to be "Datasets and Supplemental...", we should update the preface.