message: added dual example urdf as default robot for the combined control launch
diff: <launch>
  <!-- The path to a robot description (xacro) file to control -->
  - <arg name="robot" />
  + <arg name="robot" default="$(find franka_description)/robots/dual_panda_example.urdf.xacro" />
  <!-- Additional XACRO args. Be sure to escape colons and equal signs
  with backslashes, because XACRO use same syntax as roslaunch:
  + roslaunch <pkg> <launchfile> robot:=path/to/urdf.xacro args:="foo\:\=bar other\:\=true" -->
  - roslaunch <pkg> <launchfile> robot:=path/to/urdf.xacro args:="foo\:\=bar other\:\=true"
  - -->
  <arg name="args" default="" />
  <arg name="robot_id" default="panda_dual" />
  