doc: use 'an' over 'a', remove redundant sentence

PR-URL: https://github.com/nodejs/node/pull/9345


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>