ip6addrctl.8: Reference RFC 6724 instead

Commit e695500d3cb945e62b4591cb68792601ed5c4b46 updated the policy table
to match RFC 6724, which obsoletes RFC 3484.

Add a reference to RFC 6724, and mark it up as a technical report (%R).

MFC after: 3 days
Signed-off-by: Jose Luis Duran <jlduran@gmail.com>

Reviewed by: imp, glebius
Pull Request: https://github.com/freebsd/freebsd-src/pull/1375

(cherry picked from commit 3272054073720172ebae9c2c5c75e656f72a6df1)