PYCBC-229: mapkey single: remove [] in docs

Change-Id: I86dcfba25be69ab917be57d91665b9b3e751fe80
Reviewed-on: http://review.couchbase.org/35693
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>