[ietf-dkim] Issue 1575: Streamline Abstract

[Stephen Farrell]

Issue description: https://rt.psg.com/Ticket/Display.html?id=1575

Thread: http://mipassoc.org/pipermail/ietf-dkim/2008q2/010386.html

ssp-04 contains the new abstract suggested in the tracker.

Two changes were suggested on the list that aren't
included in ssp-04, I think those were:

-  s/address/address(es)/
-  s/DKIM signature/valid DKIM signature/

If there's no further discussion, I'll ask Eliot to close
this one on July 11.

S.