Update my_robot_rviz.launch

[Ktysai]

ROS NOETIC

Solution for: #1 in order to make the robot aprear corectly in RVIz and deal with this type of error message: "ERROR: cannot launch node of type [robot_state_publisher/state_publisher]: Cannot locate node of type [state_publisher] in package [robot_state_publisher]. Make sure file exists in package path and permission is set to executable (chmod +x)"

#2 necessary to solve NO GUI for joint_state_publisher when using RVIZ