[.NET] Use collection expressions in docs

As of C# 12 we can now use collection expressions to reduce some boilerplate when initializing collections.

doc/classes/Geometry2D.xml

@@ -204,7 +204,7 @@
 				print(polygon) # Prints [(50.0, 50.0), (150.0, 50.0), (150.0, 150.0), (50.0, 150.0)]
 				[/gdscript]
 				[csharp]
-				var polygon = new Vector2[] { new Vector2(0, 0), new Vector2(100, 0), new Vector2(100, 100), new Vector2(0, 100) };
+				Vector2[] polygon = [new Vector2(0, 0), new Vector2(100, 0), new Vector2(100, 100), new Vector2(0, 100)];
 				var offset = new Vector2(50, 50);
 				polygon = new Transform2D(0, offset) * polygon;
 				GD.Print((Variant)polygon); // Prints [(50, 50), (150, 50), (150, 150), (50, 150)]