Updated C# HelloWorld code block

The original code block was in a "public 2DScript()" method with no return type(or void) what so ever. I edited the C# version to be in a initializer/constructor, just like how the GDScript version is like

Author: bk0704

getting_started/step_by_step/scripting_first_script.rst

@@ -148,10 +148,10 @@ Add the following code to your script:
 
  .. code-tab:: csharp C#
 
-    public MySprite2D()
-    {
-        GD.Print("Hello, world!");
-    }
+	public override void _Ready()
+	{
+		GD.Print("Hello, world!");
+	}
 
 
 Let's break it down. The ``func`` keyword defines a new function named