Fix compile error in advanced_postprocessing.rst

tutorials/shaders/advanced_postprocessing.rst

@@ -46,6 +46,8 @@ and sets the vertex position directly.
 .. code-block:: glsl
 
   shader_type spatial;
+  // Prevent the quad from being affected by lighting and fog. This also improves performance.
+  render_mode unshaded, fog_disabled;
 
   void vertex() {
     POSITION = vec4(VERTEX, 1.0);