doc: fix broken link in dgram doc

PR-URL: #8365
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>

Author: mscdex

doc/api/dgram.md

@@ -455,6 +455,7 @@ and `udp6` sockets). The bound address and port can be retrieved using
 [`'close'`]: #dgram_event_close
 [`addMembership()`]: #dgram_socket_addmembership_multicastaddress_multicastinterface
 [`close()`]: #dgram_socket_close_callback
+[`cluster`]: cluster.html
 [`dgram.createSocket()`]: #dgram_dgram_createsocket_options_callback
 [`dgram.Socket#bind()`]: #dgram_socket_bind_options_callback
 [`Error`]: errors.html#errors_class_error