Fix API documentation for twisted.names.hosts.Resolver.lookupIPV6Address

Author: esacteksab
Reviewer: exarkun
Fixes: #6246


Fix `lookupIPV6Address` API documentation to talk about IPv6 and AAAA records instead of
IPv4 and A records.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@36825 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb