Fixed example plugin tutorial links (#638)

Signed-off-by: Alejandro Hernández Cordero <ahcorde@gmail.com>

Author: ahcorde

tutorials/03_plugins.md

@@ -20,7 +20,7 @@ Gazebo GUI will look for plugins on the following paths, in this order:
 1. All paths set on the `GZ_GUI_PLUGIN_PATH` environment variable
 2. All paths added by calling `gz::gui::addPluginPath`
 3. `~/.gz/gui/plugins`
-4. [Plugins which are installed with Gazebo GUI](https://gazebosim.org/api/gui/6.0/namespace gz_1_1gui_1_1plugins.html)
+4. [Plugins which are installed with Gazebo GUI](https://gazebosim.org/api/gui/9/namespacegz_1_1gui_1_1plugins.html)
 
 ## Configuring plugins
 
@@ -120,4 +120,4 @@ order:
 
 1. All paths set on the `GZ_GUI_DISPLAY_PLUGIN_PATH` environment variable
 2. `~/.gz/gui/display_plugins`
-3. [Display plugins that are installed with Gazebo GUI](https://gazebosim.org/api/gui/0.1/namespace gz_1_1gui_1_1display_plugins.html)
+3. [Display plugins that are installed with Gazebo GUI](https://gazebosim.org/api/gui/9/namespacegz_1_1gui_1_1plugins.html)