Add open content language and link to back cover copy on paperback

[geealbers]

From @klevinegetty:

As mentioned, after discussing POD descriptive copy with Maureen last week, I would like to add a line (set as a new paragraph) at the end of the descriptive copy and above the author bios [on the print cover]. The text should read:

Reflecting the Getty’s commitment to open content, Ancient Lamps in the J. Paul Getty Museum is also available online at http://www.getty.edu/publications/ancientlamps and may be downloaded free of charge in multiple formats.

Hopefully that matches what appears on the Artistry in Bronze back cover (title and URL adjustment notwithstanding). Let’s double check that once it is finalized.