[ros-users] Correction for PR2 tutorial wiki page

I didn't feel comfortable editing a page of this importance, but I think
there is a typo on the following Wiki page:

http://www.ros.org/wiki/pr2_controllers/Tutorials/Moving%20the%20arm%20using%20the%20Joint%20Trajectory%20Action

The command line in section 1.3 that looks like this:

rosnode list r_arm_controller

should be:

rostopic list r_arm_controller

--patrick