tutorials/segmentation_camera: Little fixes

[peci1]

🦟 Bug fix

Fixes gazebosim/gazebo_test_cases#1515

Summary

Little fixes found when going through the tutorial.

I renamed some topics so that they correspond directly to what is available in segmentation_camera.sdf so that users can directly apply what they see in the tutorial.

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.