Correct license terminology used in copyrights document

The copyrights document refers to the "Creative Commons
Attribution-ShareAlike 4.0 Unported" license. As of version 4.0, the
Creative Commons licenses have been authored in language that should be
applicable internationally rather than utilising language explicitly
written with the U.S. legal system in mind.

Previous revisions relied on ports of the license for other territories
with the original being the "unported" license. Version 4.0 discourages
using ported versions and instead acts as a single global license and
should thus be referred to as the "Creative Commons Attribution-ShareAlike
4.0 International" license.

Additionally provide a link to the license.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>