Update changelog

Author: hasufell

changelog.md

@@ -1,6 +1,6 @@
 # Changelog for [`unix` package](http://hackage.haskell.org/package/unix)
 
-## 2.8.4.0 *??? 2023*
+## 2.8.4.0 *Dec 2023*
 
   * add `haveStatx`
   * fix `statx.stx_mnt_id` detection on buggy glibc, see [GHC #24072](https://gitlab.haskell.org/ghc/ghc/-/issues/24072)