smart_arm_desc:rename right_knuckle_joint -> claw_joint

The name "right_knuckle_joint" seems a little wrong when compared with
the actual arm. The underlying servo joint on the arm is named
"claw_controller". Rename the joint to match.
Merge pull request #20 from rjw57/rename-right-knuckle-joint

smart_arm_desc:rename right_knuckle_joint -> claw_joint