doc: fix typo in dgram doc

Remove stray square brackets from dgram documentation.

PR-URL: nodejs#5114
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/dgram.markdown

@@ -86,7 +86,7 @@ Returns an object containing the address information for a socket.
 For UDP sockets, this object will contain `address`, `family` and `port`
 properties.
 
-### [socket.bind([port][, address][, callback])]
+### socket.bind([port][, address][, callback])
 
 * `port` Integer, Optional
 * `address` String, Optional