doc: "a" -> "an" in api/documentation.md

Fixes: https://github.com/nodejs/node/issues/6642
PR-URL: https://github.com/nodejs/node/pull/6689


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Alexander Makarenko <estliberitas@gmail.com>
Reviewed-By: Jeremiah Senkpiel <Fishrock123@rocketmail.com>