CHANGELOG: add description for patch 2726

Author: dannywillems

CHANGELOG.md

@@ -23,6 +23,10 @@ This project adheres to
 - Added a function `Mina.Transaction.hash(json)` to generate the hash of a JSON
   Transaction. https://github.com/o1-labs/o1js/pull/2704
 
+### Changes
+
+- Update Rust nightly version to 2025-12-11. https://github.com/o1-labs/o1js/pull/2726
+
 ## [2.12.0](https://github.com/o1-labs/o1js/compare/c2e51a84...34caaedc9) - 2025-12-03
 
 ### Fixed