Fix dead links in the Iron release notes. (ros2#3479)

That is, these demos are not available in Humble or
Foxy, so when we build the sphinx site with them, we
point at the wrong place.  Specifically point them
at the 'iron' branch, which fixes this problem.

Signed-off-by: Chris Lalancette <[email protected]>

Author: clalancette

source/Releases/Release-Iron-Irwini.rst

@@ -59,7 +59,7 @@ All of the information is published on a hidden topic generated from the name of
 So if the service is called ``/myservice``, then the information will be published on ``/myservice/_service_event``.
 
 Note that this functionality is disabled by default; to enable it, users must call ``configure_introspection`` after creating a server client or server.
-There are examples showing how to do this in https://github.com/ros2/demos/tree/{DISTRO}/demo_nodes_cpp/src/services (C++) and https://github.com/ros2/demos/blob/{DISTRO}/demo_nodes_py/demo_nodes_py/services/introspection.py (Python).
+There are examples showing how to do this in https://github.com/ros2/demos/tree/iron/demo_nodes_cpp/src/services (C++) and https://github.com/ros2/demos/blob/iron/demo_nodes_py/demo_nodes_py/services/introspection.py (Python).
 
 See `REP 2012 <https://github.com/ros-infrastructure/rep/pull/360>`__ and the tracking bug at https://github.com/ros2/ros2/issues/1285 for more information.
 
@@ -77,7 +77,7 @@ The callbacks are called in this order:
 * The "set" parameter callback, which cannot modify the list and should only accept or reject the parameters based on their type and value (this is the existing callback).
 * The "post" set parameter callback, which can make state changes based on parameters and is only called if the previous two callbacks are successful.
 
-There are examples of this in action in https://github.com/ros2/demos/blob/{DISTRO}/demo_nodes_cpp/src/parameters/set_parameters_callback.cpp (C++) and https://github.com/ros2/demos/blob/{DISTRO}/demo_nodes_py/demo_nodes_py/parameters/set_parameters_callback.py (Python).
+There are examples of this in action in https://github.com/ros2/demos/blob/iron/demo_nodes_cpp/src/parameters/set_parameters_callback.cpp (C++) and https://github.com/ros2/demos/blob/iron/demo_nodes_py/demo_nodes_py/parameters/set_parameters_callback.py (Python).
 
 See https://github.com/ros2/rclcpp/pull/1947, https://github.com/ros2/rclpy/pull/966, and https://github.com/ros2/demos/pull/565 for more information.