update the docs of the demos section

doc/index.rst

@@ -185,23 +185,14 @@ Where the parameters are as follows:
 
 The same definitions apply to the ``vel_*`` parameters.
 
-The PID parameters can be defined for ``position`` or ``position_pid`` and ``velocity`` or ``velocity_pid`` command interfaces as explained above, or definiting them in a yaml file and loading it in the ``gazebo_ros2_control`` plugin as below:
+The PID parameters can be defined for ``position`` or ``position_pid`` and ``velocity`` or ``velocity_pid`` command interfaces as explained above, or defining them in a yaml file and loading it in the ``gazebo_ros2_control`` plugin as below:
 
 .. code-block:: yaml
 
   gazebo_ros2_control:
     ros__parameters:
       pid_gains:
-        position:
-          slider_to_cart: {kp:  100.0, kd: 10.0, ki:  1.0, max_integral_error: 10000.0}
-
-
-.. code-block:: yaml
-
-  gazebo_ros2_control:
-    ros__parameters:
-      pid_gains:
-        position_pid:
+        position_pid: # (or) position
           slider_to_cart: {kp:  100.0, kd: 10.0, ki:  1.0, max_integral_error: 10000.0}
 
 .. code-block:: xml
@@ -419,10 +410,21 @@ The following examples shows a vertical cart control by a PID joint using positi
 
 .. code-block:: shell
 
+  ros2 launch gazebo_ros2_control_demos vertical_cart_example_position_pids_in_yaml.launch.py
   ros2 launch gazebo_ros2_control_demos vertical_cart_example_position_pid.launch.py
   ros2 launch gazebo_ros2_control_demos vertical_cart_example_velocity_pid.launch.py
 
 .. code-block:: shell
 
   ros2 run gazebo_ros2_control_demos example_position_pid
   ros2 run gazebo_ros2_control_demos example_velocity
+
+The ``vertical_cart_example_position_pids_in_yaml.launch.py`` example uses a yaml file as following to set the PID gains:
+
+.. code-block:: yaml
+
+  gazebo_ros2_control:
+    ros__parameters:
+      pid_gains:
+        position:
+          slider_to_cart: {kp:  100.0, kd: 10.0, ki:  1.0, max_integral_error: 10000.0}