doc: fix some table problems in changelog.md

PR-URL: https://github.com/nodejs/node/pull/9183


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>