doc: a listener, not "an" listener
doc: a listener, not "an" listener

Reviewed-By: Sam Roberts <266bbf644f3674ab209c4709d090256bac1c7bb7@gmail.com>
Reviewed-By: Trevor Norris <9cb5491eb5770dd8e0bf77a8a0335abf5f0488c5@gmail.com>
PR-URL: https://github.com/nodejs/io.js/pull/1025

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node