doc: fix a link in dgram.md

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, dgram

Refs: https://en.wikipedia.org/w/index.php?title=IPv6_address&type=revision&diff=809494791&oldid=804196124