api3/touch: command is const

Change-Id: Ie8f6c16d3a80d72d45046c8f3507a8a8ace9b51e
Reviewed-on: http://review.couchbase.org/38935
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>