Update whatsnew for 3.3.1 release (#5002)

* set whatsnew release date

* fix whatsnew

Author: stephenworsley

docs/src/whatsnew/3.3.rst

@@ -31,8 +31,8 @@ This document explains the changes made to Iris for this release
    any issues or feature requests for improving Iris. Enjoy!
 
 
-v3.3.1 |build_date| [unreleased]
-================================
+v3.3.1 (29 Sep 2022)
+====================
 
 .. dropdown:: :opticon:`alert` v3.3.1 Patches
    :container: + shadow