Update custom_postprocessing.rst

I just think a little more detail could be added on where to find some of these settings in the nodes. If you have any suggestions on how it could be worded better let me know.

Author: ShinyAdmiral

tutorials/shaders/custom_postprocessing.rst

@@ -23,7 +23,7 @@ Post-processing effects are shaders applied to a frame after Godot has rendered
 it. To apply a shader to a frame, create a :ref:`CanvasLayer
 <class_CanvasLayer>`, and give it a :ref:`ColorRect <class_ColorRect>`. Assign a
 new :ref:`ShaderMaterial <class_ShaderMaterial>` to the newly created
-``ColorRect``, and set the ``ColorRect``'s layout to "Full Rect".
+``ColorRect``, and set the ``Anchors Preset`` to "Full Rect" located in ``ColorRect``'s layout.
 
 Your scene tree will look something like this: