README.md: update to reflect new ROS package prefix
README.md: update to reflect new ROS package prefix

Update the setup instructions to reflect the use of ${ros_prefix}.

rapyuta/meta-ros,simium/meta-ros,bulwahn/meta-ros,bmwcarit/meta-ros,bmwcarit/meta-ros,simium/meta-ros,bulwahn/meta-ros,bulwahn/meta-ros,bmwcarit/meta-ros,rapyuta/meta-ros,rapyuta/meta-ros,KristofRobot/meta-ros,KristofRobot/meta-ros,bmwcarit/meta-ros,simium/meta-ros