doc: update io.js reference

Commit f3375954 ("lib,src: add unix socket getsockname/getpeername")
accidentally left in a reference to io.js v1.5.0 in the documentation.
Update it to say "Node.js v4.0.0", the upcoming release.

PR-URL: https://github.com/nodejs/node/pull/2580


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>