Correct typos in .set documentation.

Change-Id: Ic0efe48bee72be0d4d71b650e5b7e5d95e8a1b19
Reviewed-on: http://review.couchbase.org/28743
Tested-by: Brett Lawson <brett19@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>