Fix a link in the underwater vehicle tutorial

[iche033]

🦟 Bug fix

Summary

Link was broken. This fixes it to correctly point to the gazebo-classic 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)
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[arjo129]

Ideally we should point to the correct new gazebo system.