Update instancing example for Godot 4

[Piralein]

• removed the default_env and replaced the icon.png with icon.svg
• updated the Main.gd script with new syntax

[andreymal]

As in godotengine/godot-docs#6187, collision shapes are still smaller in Godot 4

[Piralein]

As in godotengine/godot-docs#6187, collision shapes are still smaller in Godot 4

fixed.

[skyace65]

Tested the project, works perfectly for me. Thanks!