doc: fixed typo in net.markdown (missing comma)

per: https://github.com/joyent/node/pull/7930

originally submitted by @pbrewczynski

Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2378

doc/api/net.markdown

@@ -334,7 +334,7 @@ Construct a new socket object.
 
 `options` is an object with the following defaults:
 
-    { fd: null
+    { fd: null,
       allowHalfOpen: false,
       readable: false,
       writable: false