fix fortress docs (gazebosim#423)

* Update gazebo configuration for downward movement

* update the correct path for rrbot.urdf file

Author: khaledgabr77

fortress/moving_robot.md

@@ -160,7 +160,7 @@ Map each arrow (key stroke) with the desired message (movement) as we did with t
 * Left ➞ 16777234 ➞ linear: {x: 0.0}, angular: {z: 0.5}
 * Up ➞ 16777235 ➞ linear: {x: 0.5}, angular: {z: 0.0}
 * Right ➞ 16777236 ➞ linear: {x: 0.0}, angular: {z: -0.5}
-* Down ➞ 16777237 ➞ linear: {x: 0.5}, angular: {z: 0.0}
+* Down ➞ 16777237 ➞ linear: {x: -0.5}, angular: {z: 0.0}
 
 Now it's your turn try to make the robot move using different keys.
 

fortress/spawn_urdf.md

@@ -8,7 +8,7 @@ More information about URDF can be found at [https://wiki.ros.org/urdf](https://
 ## Obtaining a URDF file
 
 This tutorial assumes that the reader already has a URDF file that they'd like to load into Gazebo Sim.
-If you are looking for a URDF file to use, you can use the [`rrbot.urdf`](tutorials/spawn_urdf/rrbot.urdf) that was preprocessed for you. The `rrbot` robot model is from the [`gazebo_ros_demos`](https://github.com/ros-simulation/gazebo_ros_demos) package. You could also use one of the examples listed at [https://wiki.ros.org/urdf/Examples](https://wiki.ros.org/urdf/Examples).
+If you are looking for a URDF file to use, you can use the [`rrbot.urdf`](https://github.com/gazebosim/docs/blob/master/fortress/tutorials/spawn_urdf/rrbot.urdf) that was preprocessed for you. The `rrbot` robot model is from the [`gazebo_ros_demos`](https://github.com/ros-simulation/gazebo_ros_demos) package. You could also use one of the examples listed at [https://wiki.ros.org/urdf/Examples](https://wiki.ros.org/urdf/Examples).
 
 If you have a `xacro` representation of a robot model, you can turn the `xacro` file into a `URDF` file using the [`xacro`](https://index.ros.org/p/xacro/) package: see [this tutorial](https://docs.ros.org/en/foxy/Tutorials/URDF/Using-Xacro-to-Clean-Up-a-URDF-File.html) for more information.