doc: label v4.2.1 as LTS in changelog heading

PR-URL: #3360
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

Author: phillipj

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Node.js ChangeLog
 
-## 2015-10-13, Version 4.2.1 'Argon' (Stable), @jasnell
+## 2015-10-13, Version 4.2.1 'Argon' (LTS), @jasnell
 
 ### Notable changes