doc: fix a link in dgram.md

PR-URL: https://github.com/nodejs/node/pull/17107
Refs: https://en.wikipedia.org/w/index.php?title=IPv6_address&type=revision&diff=809494791&oldid=804196124


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>