Fix Servo pose tracking tutorial

[sea-bass]

Description

The Servo pose tracking tutorial was not working as it had likely gone out of date with some other changes.

Also, the way the command switching service was called wasn't properly waiting for the future to complete, which caused me problems. It still doesn't work reliably on FastDDS, but if you switch RMW to CycloneDDS it does.

Checklist

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[sea-bass]

@ibrahiminfinite Also tagging you here, if you're interested in giving it a test