Add in a link to the composition demo launch file. (#3553)

This makes it easier for people to see what a launch file
with composition should look like.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

source/Tutorials/Intermediate/Composition.rst

@@ -194,7 +194,7 @@ Composition using launch actions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 While the command line tools are useful for debugging and diagnosing component configurations, it is frequently more convenient to start a set of components at the same time.
-To automate this action, we can use the functionality in ``ros2 launch``.
+To automate this action, we can use a `launch file <https://github.com/ros2/demos/blob/{REPOS_FILE_BRANCH}/composition/launch/composition_demo_launch.py>`__:
 
 .. code-block:: bash