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: https://github.com/nodejs/node/pull/7394


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