Fixes apply actions method in the `NonHolonomicAction` action term class (#317)

# Description

in `NonHolonomicAction.apply_actions()` the velocity command was
incorrectly named `self.joint_vel` instead of `self._joint_vel_command`
as initialized in `NonHolonomicAction.__init__()`. This causes an error
when trying to instantiate the action config. I have corrected
and tested that it works.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [X] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [X] I have made corresponding changes to the documentation
- [X] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [X] I have run all the tests with `./orbit.sh --test` and they pass
- [X] 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

source/extensions/omni.isaac.orbit/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.15.3"
+version = "0.15.4"
 
 # Description
 title = "ORBIT framework for Robot Learning"

source/extensions/omni.isaac.orbit/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.15.4 (2024-03-22)
+~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Fixed the NonHolonomicActionCfg variable naming from joint_vel to _joint_vel_command to match the initialized variable in the init() function.
+
+
 0.15.3 (2024-03-21)
 ~~~~~~~~~~~~~~~~~~~
 
@@ -11,6 +20,7 @@ Added
 
 Fixed
 ^^^^^
+
 * Moved class variables in :class:`omni.isaac.orbit.scene.InteractiveScene` to correctly  be assigned as
   instance variables.
 * Removed custom ``__del__`` magic method from :class:`omni.isaac.orbit.scene.InteractiveScene`.

source/extensions/omni.isaac.orbit/omni/isaac/orbit/envs/mdp/actions/non_holonomic_actions.py

@@ -134,8 +134,8 @@ class NonHolonomicAction(ActionTerm):
         quat_w = self._asset.data.body_quat_w[:, self._body_idx]
         yaw_w = euler_xyz_from_quat(quat_w)[2]
         # compute joint velocities targets
-        self.joint_vel[:, 0] = torch.cos(yaw_w) * self.processed_actions[:, 0]  # x
-        self.joint_vel[:, 1] = torch.sin(yaw_w) * self.processed_actions[:, 0]  # y
-        self.joint_vel[:, 2] = self.processed_actions[:, 1]  # yaw
+        self._joint_vel_command[:, 0] = torch.cos(yaw_w) * self.processed_actions[:, 0]  # x
+        self._joint_vel_command[:, 1] = torch.sin(yaw_w) * self.processed_actions[:, 0]  # y
+        self._joint_vel_command[:, 2] = self.processed_actions[:, 1]  # yaw
         # set the joint velocity targets
-        self._asset.set_joint_velocity_target(self.joint_vel, joint_ids=self._joint_ids)
+        self._asset.set_joint_velocity_target(self._joint_vel_command, joint_ids=self._joint_ids)