Update Instantiate menu in 03.player_movement_code.rst (godotengine#7978)

This commit updates the instructions for instantiating a new child
scene in the getting started first 3d game tutorial so that the
menu option matches the language used in version 4.1.1.

The associated image is updated to match and is updated to the
preferred `webp` format.

The "Instance Child Scene" menu now reads "Instantiate Child Scene".

Author: manik1235

getting_started/first_3d_game/03.player_movement_code.rst

@@ -361,7 +361,7 @@ tab at the top of the editor to do so.
 If you closed the scene before, head to the *FileSystem* dock and double-click
 ``main.tscn`` to re-open it.
 
-To instantiate the ``Player``, right-click on the ``Main`` node and select *Instance
+To instantiate the ``Player``, right-click on the ``Main`` node and select *Instantiate
 Child Scene*.
 
 |image2|
@@ -446,7 +446,7 @@ work on the monsters.
 
 .. |image0| image:: img/03.player_movement_code/01.attach_script_to_player.webp
 .. |image1| image:: img/03.player_movement_code/02.clicking_main_tab.png
-.. |image2| image:: img/03.player_movement_code/03.instance_child_scene.png
+.. |image2| image:: img/03.player_movement_code/03.instance_child_scene.webp
 .. |image3| image:: img/03.player_movement_code/04.scene_tree_with_camera.png
 .. |image4| image:: img/03.player_movement_code/05.camera_preview_checkbox.png
 .. |image5| image:: img/03.player_movement_code/06.two_viewports.png