Update 'Motion planning for a MoveIt 2 arm' link on Demos page (ros2#3619)

Signed-off-by: Christophe Bedard <[email protected]>

Author: christophebedard-apexai

source/Tutorials/Demos.rst

@@ -17,7 +17,7 @@ External resources
 ------------------
 
 * `Bridging communication between ROS 1 and ROS 2 <https://github.com/ros2/ros1_bridge/blob/master/README.md>`__
-* `Motion planning for a MoveIt 2 arm <http://moveit2_tutorials.picknik.ai/>`__
+* `Motion planning for a MoveIt 2 arm <https://moveit.picknik.ai/main/index.html>`__
 * Using Turtlebot 3 (community-contributed)
 
     - `Getting started <https://emanual.robotis.com/docs/en/platform/turtlebot3/quick-start/>`__