[doc] Fix the type of the socket argument

Author: lpinca

doc/ws.md

@@ -249,8 +249,7 @@ receives an `Error` if the server is already closed.
 ### server.handleUpgrade(request, socket, head, callback)
 
 - `request` {http.IncomingMessage} The client HTTP GET request.
-- `socket` {net.Socket|tls.Socket} The network socket between the server and
-  client.
+- `socket` {stream.Duplex} The network socket between the server and client.
 - `head` {Buffer} The first packet of the upgraded stream.
 - `callback` {Function}.