Fix non-unique Sphinx name in Getting Started (#5597)

getting_started/first_2d_game/index.rst

@@ -64,7 +64,7 @@ Contents
 
 .. toctree::
    :maxdepth: 1
-   :name: toc-learn-introduction
+   :name: toc-learn-first_2d_game
 
    01.project_setup
    02.player_scene

getting_started/first_3d_game/index.rst

@@ -54,7 +54,7 @@ Contents
 
 .. toctree::
    :maxdepth: 1
-   :name: toc-learn-introduction
+   :name: toc-learn-first_3d_game
 
    01.game_setup
    02.player_input