Update harmonic/ros2_spawn_model.md

Co-authored-by: Addisu Z. Taddese <[email protected]>
Signed-off-by: Carlos Agüero <[email protected]>

Author: caguero

harmonic/ros2_spawn_model.md

@@ -11,7 +11,7 @@ The package `ros_gz_sim` contains a launch file named
 existing simulation. Here's an example:
 
 ```bash
-ros2 launch ros_gz_sim gz_spawn_model.launch.py world:=empty file:=/home/caguero/ros_gz_ws/src/ros_gz/ros_gz_sim_demos/models/vehicle/model.sdf name:=my_vehicle x:=5.0 y:=5.0 z:=0.5
+ros2 launch ros_gz_sim gz_spawn_model.launch.py world:=empty file:=$(ros2 pkg prefix --share ros_gz_sim_demos)/models/vehicle/model.sdf name:=my_vehicle x:=5.0 y:=5.0 z:=0.5
 ```
 
 Check [this block](https://github.com/gazebosim/ros_gz/blob/cadae1c8323a74395c09a37e3de4c669c8c09d4f/ros_gz_sim/launch/ros_gz_spawn_model.launch.py#L33-L44)