Added a small comment to post_processing example with instructions on how to make it work for 2d (#12775)

# Objective

- `examples/shader/post_processing.rs` is a shader example that works
for 3d
- I recently tried to update this example to get it to work on 2d but
failed to do so
- Then I created a discord help thread to help me figure this out.
[here's the link to the
thread](https://discordapp.com/channels/691052431525675048/1221819669116354723).

## Solution

- The solution is to replace all instances of 3d structures with their
respective 2d counterparts

## Changelog

- Added a small comment that explains how to get the example to work on
2d


#### Please do suggest changes if any

---------

Co-authored-by: IceSentry <IceSentry@users.noreply.github.com>

examples/shader/post_processing.rs

@@ -2,6 +2,7 @@
 //! and reads the texture generated by the main pass.
 //!
 //! The example shader is a very simple implementation of chromatic aberration.
+//! To adapt this example for 2D, replace all instances of 3D structures (such as `Core3D`, etc.) with their corresponding 2D counterparts.
 //!
 //! This is a fairly low level example and assumes some familiarity with rendering concepts and wgpu.