Update changelog

Author: mauvilsa

src/lightning/pytorch/CHANGELOG.md

@@ -8,7 +8,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
-- Allow LightningCLI to use a customized argument parser class ([#???](https://github.com/Lightning-AI/pytorch-lightning/pull/???))
+- Allow LightningCLI to use a customized argument parser class ([#20596](https://github.com/Lightning-AI/pytorch-lightning/pull/20596))
 
 ### Changed