Plugins docs on Dotnet C# EditorInterface to get singleton is wrong and misleading

[DamienFremont]

Your Godot version:
4.3, 4.2

Issue description:

Compile error using C# on godot-demo-projects:plugins:main_screen
https://github.com/DamienFremont/godot-demo-projects/tree/master/plugins/addons/main_screen

Plugin script should be using EditorInterface.singleton. instead of EditorInterface.

URL to the documentation page:
https://docs.godotengine.org/en/latest/tutorials/plugins/editor/making_main_screen_plugins.html

Fix + PR:
#9627

Test project
https://github.com/DamienFremont/godot-demo-projects/tree/fix-docs-dotnet-editorinterface-singleton-usage/plugins

Screenshots:

[AThousandShips]

This is not wrong, see #9399 (comment), though it's one of two different ways of doing this, however neither is more or less correct

[GeneralProtectionFault]

I ran into this and came to see if there was an issue, and there was =D.
@AThousandShips What is it that's not wrong, though? This isn't about which way to implement, it's about the documentation, right?

The C# code example on the referenced documentation is wrong--outright won't build. I'm assuming the proposal is simply replace this:
public override Texture2D _GetPluginIcon() { return EditorInterface.GetEditorTheme().GetIcon("Node", "EditorIcons"); }

...with this:
public override Texture2D _GetPluginIcon() { return EditorInterface.Singleton.GetEditorTheme().GetIcon("Node", "EditorIcons"); }

It's in two spots on the docs page.