Use the ROS standard way for namespaces

[horverno]

This is not the correct way to use namespaces:

https://github.com/jkk-research/lexus_bringup/blob/5fec6fb4f1efa3c78170586236dda93eab3b363d/src/speed_control.cpp#L37-L49

Rather:

return LaunchDescription([
        Node(
            package='lexus_bringup',
            namespace='lexus3', ## ADD
            executable='speed_control',
            name='speed_control_pid_longitudinal',
            parameters=[
                {"p_gain_accel": 0.29},
                {"i_gain_accel": 0.035},
                {"d_gain_accel": 0.0},
                {"p_gain_brake": 0.15},
                {"i_gain_brake": 0.038},
                {"d_gain_brake": 0.0},
                {"lx_namespace": "lexus3"} ## REMOVE
        ]
        )
    ])

Similarly to this