omitted DOI for ggplot2

[Cory Brunson]

JOSS prefers that references include DOIs if possible. Most of the references without DOIs are R packages or blog posts (and don't have DOIs), but Wickham's book on ggplot2 is cited as a book and therefore could helpfully include the DOI of the Springer–Verlag edition.

Part of this JOSS review.

[Robert Garrett]

Added the doi