Fix rgba value in oven_asset.xml to be in [0,1]

[RussTedrake]

Description

The mujoco xml docs state that "All [rgba] components should be in the range [0 1]."
https://mujoco.readthedocs.io/en/stable/XMLreference.html#asset-material

MuJoCo's parser clearly let's this through. But it seems that mine is a little more strict.

I checked with the original author @abhishekunique, and he confirmed that it's just a bug (from his original work), and that it should be fixed.

Related to RobotLocomotion/drake#22445.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes