doc: fix layout problem in v4 changelog

The current layout is breaking the release post tool.

This commit also removed erroneous entires in the main CHANGELOG for
v4.4.6 and v5.12.0.

PR-URL: #7394
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## 2016-06-22, Version 4.4.6 'Argon' (LTS), @thealphanerd
 
+### Notable Changes
+
 This is an important security release. All Node.js users should consult the security release summary at nodejs.org for details on patched vulnerabilities.
 
 This release is specifically related to a Buffer overflow vulnerability discovered in v8, more details can be found [in the CVE](https://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-1669)