doc: fix http.IncomingMessage.socket documentation

Remove the reference to net.Socket.verifyPeer().
That was removed in ea540c94 and was missed in the 032f80ef.

Refer to the net.Socket instance by the .socket property.
This avoids unneeded confusion.
'.socket' is the variant that is used internally.

Add a markdown link to net.Socket.getPeerCertificate().

PR-URL: https://github.com/nodejs/io.js/pull/1867


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>