Fix PDActuator docstring to match actual implementation

[lorenwel]

Description

The docstring of the IdealPDActuator didn't match its implementation. Desired and actual joint positions and velocities were swapped.

Actual implementation is like this:

def compute(
        self, control_action: ArticulationActions, joint_pos: torch.Tensor, joint_vel: torch.Tensor
    ) -> ArticulationActions:
        # compute errors
        error_pos = control_action.joint_positions - joint_pos
        error_vel = control_action.joint_velocities - joint_vel
        # calculate the desired joint torques
        self.computed_effort = self.stiffness * error_pos + self.damping * error_vel + control_action.joint_efforts

It is "desired - current", the current docstring says the opposite:

Type of change

• Documentation update

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[jtigue-bdai]

Thanks, good catch

[lorenwel]

@Mayankm96 @jtigue-bdai Thanks for the review.
The code build step didn't run so I cannot merge this. How do I get it to trigger?