Fix custom sensor system example build

[iche033]

🦟 Bug fix

Summary

The Plugins/ExamplesBuild test started failing since we bumped the version in gz-sensors's main branch. This PR updates the custom sensor example's CMakeLists to checkout gz-sensors9 instead of main

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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