Update URDF model link to new repository path (backport #6772)

[mergify]

Description

The documentation currently links to the master branch of the urdf_tutorial
repository for the 07-physics.urdf example file. The master branch contains
incorrect inertia values (ixx="1.0", iyy="1.0", izz="1.0") which is the
identity matrix. The documentation itself warns against using the identity matrix:

"The identity matrix is a particularly bad choice, since it is often much too
high (it corresponds to a box of 0.1 m side length with a mass of 600 kg!)"

The ros2 branch of urdf_tutorial already has the correct values
(ixx="1e-3", iyy="1e-3", izz="1e-3") which is consistent with the
documentation recommendation:

"a matrix with ixx/iyy/izz=1e-3 or smaller is often a reasonable default
for a mid-sized link"

This PR updates the link to point to the ros2 branch instead of master
so that users following the tutorial see the correct example file that is
consistent with the documentation text.

Related: ros2/lyrical_tutorial_party#3157

Did you use Generative AI?

No

Additional Information

This is a minimal one-line change. The fix in the ros2 branch of
urdf_tutorial was already present — this PR simply updates the
documentation to link to the correct branch.

cc - @kscottz

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This is an automatic backport of pull request #6772 done by Mergify.

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/25906445716/artifacts/7012486005.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-6773/index.html in your favorite browser