mjcf -> SDFormat: minor fixes

[scpeters]

🦟 Bug fix

Two minor fixes for things I noticed during code review

Summary

• https://github.com/gazebosim/gz-mujoco/commit/cf34f52e6ac43811102d6f73041816bed84b1371: fix syntax for example mjcf2sdformat command in README
• https://github.com/gazebosim/gz-mujoco/commit/f1fdecedfa3a3bd8605fbc45c1938356371354e0: add unnamed_ prefix to collisions and visuals without a name to match the behavior for unnamed joints.

Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

Merging #59 (f1fdece) into main (60f2d21) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files          16       16           
  Lines         646      646           
=======================================
  Hits          632      632           
  Misses         14       14           

Impacted Files	Coverage Δ
...o_sdformat/mjcf_to_sdformat/converters/geometry.py	95.94% <100.00%> (ø)

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