[tricycle] Rename variable in launch file (backport #327)

[mergify]

Fix the variable name, possibly from some copy-pasting.

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This is an automatic backport of pull request #327 done by Mergify.

[mergify]

Cherry-pick of cd0b002 has failed:

On branch mergify/bp/iron/pr-327
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	both modified:   gz_ros2_control_demos/launch/tricycle_drive_example.launch.py

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