Update copyright year on the documentation footer

Change-Id: I911d45a32daf476e4416f3e6b9066f769fc2000d
Reviewed-on: http://review.couchbase.org/41316
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>