Typo error amde in chapter 3 creating a launch file for the mobile manipulator

At the "Simulating and testing the mobile manipulator" section of the book, there is a typo(?) made when creating a launch file:

```bash
$ cd ~/chapter3_ws/src/robot_description/launch
$ gedit mobile_manipulator_gazebo_control_xacro.launch
```

I think the `mobile_manipulator_gazebo_control_xacro.launch` should be `mobile_manipulator_gazebo_xacro.launch`

Is that correct?