Add pull request to changelog

Author: plemarquand

Release Notes/600.md

@@ -101,6 +101,7 @@
 
 - `TokenSyntax.identifier`
   - Description: Adds an `identifier` property to `TokenSyntax` which returns a canonicalized representation of an identifier that strips away backticks.
+  - Pull request: https://github.com/apple/swift-syntax/pull/2576
 
 ## API Behavior Changes