doc: Update net.markdown

Associates link to dns.lookup() with proper URL.

Fixes: https://github.com/joyent/node/issues/8018
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>

doc/api/net.markdown

@@ -603,3 +603,4 @@ Returns true if input is a version 6 IP address, otherwise returns false.
 [server.getConnections()]: #net_server_getconnections_callback
 [Readable Stream]: stream.html#stream_readable_stream
 [stream.setEncoding()]: stream.html#stream_stream_setencoding_encoding
+[dns.lookup()]: dns.html#dns_dns_lookup_domain_family_callback