fixing typo in ros-unity service tutorial (#198)

tutorials/ros_unity_integration/service.md

         {
             Debug.Log("Destination reached.");
 
-            PosRot cubePos = new PosRot(
+            MPosRot cubePos = new MPosRot(
                 cube.transform.position.x,
                 cube.transform.position.y,
                 cube.transform.position.z,