message: Update test_two_kinects_one_map.launch
diff: already created with one of the Kinect using mapping tutorial:
  http://wiki.ros.org/rtabmap_ros/Tutorials/HandHeldMapping
  $ roslaunch freenect_launch freenect.launch depth_registration:=true
  - $ roslaunch rtabmap_ros rtabmap.launch rtabmap_args:="-.-delete_db_on_start"
  + $ roslaunch rtabmap_ros rtabmap.launch args:="-d"
  -->
  