Rover: RC calibration should say ".. and throttle is centered"

Near the end of the RC calibration procedure, a messages box pops up saying, "Ensure all your sticks are centered and throttle is *down*, and click ok to continue" but for a rover, the throttle should be centered.

Locate the RC calibration procedure and the message box described in the issue, then check how the vehicle type is available at that point. Confirm the wording for Rover says the throttle is centered while other vehicle instructions remain correct, and verify the calibration flow still reaches its next step.

Written by the indexing model from the issue text.