Improve C# GD.Load() examples

* Use typed version of GD.Load()
* Use absolute paths (i.e., prefixed with "res://")
* Use "Path/To/" intermediate path values on abstract examples, to
  illustrate that files can live in various directories
* Use PascalCase on files that are not GDScript or GLSL

tutorials/physics/using_character_body_2d.rst

@@ -337,7 +337,7 @@ uses the mouse pointer. Here is the code for the Player, using ``move_and_slide(
 
     public partial class MyCharacterBody2D : CharacterBody2D
     {
-        private PackedScene _bullet = (PackedScene)GD.Load("res://bullet.tscn");
+        private PackedScene _bullet = GD.Load<PackedScene>("res://Bullet.tscn");
         private int _speed = 200;
 
         public void GetInput()