Update ros2_control_demos dependency to foxy branch (#200)

Author: fmauch

Universal_Robots_ROS2_Driver-not-released.foxy.repos

@@ -2,7 +2,7 @@ repositories:
   ros2_control_demos:
     type: git
     url: https://github.com/ros-controls/ros2_control_demos.git
-    version: master
+    version: foxy
   ur_msgs:
     type: git
     url: https://github.com/destogl/ur_msgs.git