Fix broken link to URDF spec (ros2#2189) (ros2#2190)

Likely also has to be fixed in other branches.

(cherry picked from commit 45b5556)

Co-authored-by: Andreas Bihlmaier <[email protected]>

Author: mergify[bot]

source/Tutorials/URDF/Adding-Physical-and-Collision-Properties-to-a-URDF-Model.rst

@@ -134,7 +134,7 @@ If not specified, these coefficients default to zero.
 Other Tags
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 In the realm of pure URDF (i.e. excluding Gazebo-specific tags), there are two remaining tags to help define the joints: calibration and safety controller.
-Check out the `spec <urdf/XML/joint>`_, as they are not included in this tutorial.
+Check out the `spec <https://wiki.ros.org/urdf/XML/joint>`_, as they are not included in this tutorial.
 
 Next Steps
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