Publish correct joint velocities in r6bot example 7 (#430)

Co-authored-by: Erik Johnson <erikjohnson@blackberry.com>
(cherry picked from commit 7ddc8d3)

example_7/reference_generator/send_trajectory.cpp

@@ -81,7 +81,7 @@ int main(int argc, char ** argv)
  trajectory_point_msg.positions.data(), joint_positions.data.data(),
  trajectory_point_msg.positions.size() * sizeof(double));
  std::memcpy(
- trajectory_point_msg.velocities.data(), joint_positions.data.data(),
+ trajectory_point_msg.velocities.data(), joint_velocities.data.data(),
  trajectory_point_msg.velocities.size() * sizeof(double));
 
  // integrate joint velocities