Update CHANGELOG.md to remove vNEXT from v2.4.0 header.

This was inadvertently left in place as I didn't run into the usual
merge-conflict that I usually do on the CHANGELOG.md which is my natural
ergonomic which usually leads me to fixing it.  Apologies!

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Changelog
 
-### vNEXT (v2.4.0)
+### vNEXT
+
+### v2.4.0
 
 - Implement an in-memory cache store to save parsed and validated documents and provide performance benefits for repeat executions of the same document. [PR #2111](https://github.com/apollographql/apollo-server/pull/2111) (`>=2.4.0-alpha.0`)
 - Fix: Serialize arrays as JSON on fetch in `RESTDataSource`. [PR #2219](https://github.com/apollographql/apollo-server/pull/2219)