Fix SDFormat xml output of sdf_exporter

[azeey]

🦟 Bug fix

Fixes gazebosim/gazebo_test_cases#207

Summary

The warnings were caused by improper xml hierarchy.

Checklist

• 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)
• codecheck passed (See contributing)
• 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]

Codecov Report

Merging #2156 (509f20f) into gz-sim7 (9d79b62) will not change coverage.
The diff coverage is n/a.

❗ Current head 509f20f differs from pull request most recent head 87c21d5. Consider uploading reports for the commit 87c21d5 to get more accurate results

@@           Coverage Diff            @@
##           gz-sim7    #2156   +/-   ##
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  Coverage    65.01%   65.01%           
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  Files          356      356           
  Lines        28932    28932           
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  Hits         18809    18809           
  Misses       10123    10123           

[azeey]

Looks like INTEGRATION_apply_link_wrench_system is flaky on Github Actions. /cc @Henrique-BO. I'll go ahead and merge since this only touches examples.