doc: update io.js reference

Commit f337595 ("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>

doc/api/net.markdown

@@ -256,7 +256,7 @@ Example:
 Don't call `server.address()` until the `'listening'` event has been emitted.
 
 This method used to return the file path as a string for UNIX sockets and
-Windows pipes.  As of io.js v1.5.0, it returns the expected object.
+Windows pipes.  As of Node.js v4.0.0, it returns the expected object.
 
 ### server.unref()