Tweaks to python docs

[chapulina]

🦟 Bug fix

• Follow up to Remove internal python bindings for sdformat #1447

Summary

• Removed sdformat reference from example
• Updated example and tutorials to use "test fixture" instead of "helper fixture"

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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.

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