doc: fix links order in Buffer doc

Sort links in lexical order

PR-URL: https://github.com/nodejs/node/pull/5076


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>