tutorials/06_python_support: Fix tutorial snippet

[peci1]

🦟 Bug fix

Fixes gazebosim/gazebo_test_cases#1545

Summary

Fixed tutorial snippet so that it works out of the box

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)
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