gui_system_plugin: clarify description in README (#2253)

* Clarfied description that was not clear due to rename Ignition to Gazebo.

Signed-off-by: Johan Rutgeerts <johan.rutgeerts@lancewood.eu>

* Slight rephrasing for extra clarity.

Signed-off-by: Johan Rutgeerts <johan.rutgeerts@lancewood.eu>

---------

Signed-off-by: Johan Rutgeerts <johan.rutgeerts@lancewood.eu>
Co-authored-by: Addisu Z. Taddese <addisu@openrobotics.org>
Co-authored-by: Ian Chen <ichen@openrobotics.org>

Author: 3 people

examples/plugin/gui_system_plugin/README.md

@@ -2,10 +2,10 @@
 
 This example shows how to create a GUI system plugin.
 
-Gazebo supports any kind of Gazebo GUI plugin
-(`gz::gui::Plugin`). Gazebo GUI plugins are a special type of Gazebo
-GUI plugin which also have access to entity and component updates coming from
-the server.
+Gazebo supports any kind of GUI plugin
+(`gz-gui` library: `gz::gui::Plugin`). However, GuiSystem plugins
+(`gz-sim` library: `gz::sim::GuiSystem`) are a special type of GUI plugin,
+which also have access to entity and component updates coming from the server.
 
 See `GuiSystemPluginPlugin.hh` for more information.