doc: remove extra indentation in iojs changelog

This removes the extra two spaces indentation which was somewhy present
in the iojs changelog from v1.8.1 to v3.3.1.

iojs changelog was the only file affected.

PR-URL: nodejs#7727
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>