Add missing header

doc/source/ref-changelog.md

@@ -18,6 +18,8 @@ We would like to give our special thanks to all the contributors who made the ne
 
 `Adam Narozniak`, `Charles Beauville`, `Chong Shen Ng`, `Daniel J. Beutel`, `Daniel Nata Nugraha`, `Heng Pan`, `Javier`, `Mahdi Beitollahi`, `Robert Steiner`, `Taner Topal`, `Yan Gao`, `bapic`, `mohammadnaseri` <!---TOKEN_v1.9.0-->
 
+### What's new?
+
 - **Introduce built-in authentication (preview)** ([#2946](https://github.com/adap/flower/pull/2946), [#3388](https://github.com/adap/flower/pull/3388), [#2948](https://github.com/adap/flower/pull/2948), [#2917](https://github.com/adap/flower/pull/2917), [#3386](https://github.com/adap/flower/pull/3386), [#3308](https://github.com/adap/flower/pull/3308), [#3001](https://github.com/adap/flower/pull/3001), [#3409](https://github.com/adap/flower/pull/3409), [#2999](https://github.com/adap/flower/pull/2999), [#2979](https://github.com/adap/flower/pull/2979), [#3389](https://github.com/adap/flower/pull/3389), [#3503](https://github.com/adap/flower/pull/3503), [#3366](https://github.com/adap/flower/pull/3366), [#3357](https://github.com/adap/flower/pull/3357))
 
   Flower 1.9 introduces the first build-in version of client node authentication. In previous releases, users often wrote glue code to connect Flower to external authentication systems. With this release, the SuperLink can authenticate SuperNodes using a built-in authentication system. A new [how-to guide](https://flower.ai/docs/framework/how-to-authenticate-supernodes.html) and a new [code example](https://github.com/adap/flower/tree/main/examples/flower-authentication) help you to get started.