Update custom sensor system example to make the sensor respect update rate

[iche033]

🦟 Bug fix

Fixes #2290

Summary

In the custom sensor example, the sensor calls its own Update function on every PostUpdate callback, which results in the sensor updating and publishing at 1000Hz. This PR updates the call to the base Sensor class's Update function which has logic to throttle the updates.

To test, build the example following the README instructions, run the world with the custom sensor system, and check the topic frequency:

gz sim -v 4 odometer.sdf
gz topic -f -t /world/odometer_world/model/model_with_sensor/link/link/sensor/an_odometer/odometer

It should drop down to 30Hz

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

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