doc: fix http.IncomingMessage.socket documentation

doc: fix http.IncomingMessage.socket documentation

Remove the reference to net.Socket.verifyPeer().
That was removed in ea540c9 and was missed in the 032f80e.

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>