doc: label v4.2.0 as LTS in changelog heading

Update the label for v4.2.0 to (LTS) from (Stable)

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3343

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Node.js ChangeLog
 
-## 2015-10-07, Version 4.2.0 'Argon' (Stable), @jasnell
+## 2015-10-07, Version 4.2.0 'Argon' (LTS), @jasnell
 
 ### Notable changes