[SDP-US] profile designator

The profile designator used in the current ED is inconsistent:

   - sections 3 and 4.2 specify the use of
   http://www.w3.org/TR/profile/simple-delivery
   - example 3 uses http://www.w3.org/TR/profile/online-delivery

Beyond this, I'm thinking the designation 'simple-delivery' is too general,
and should be changed to 'sdp-us' to better match the short name and scope
of this profile.

So I propose we change the profile name to "SDP US" in the table in Section
3 and the profile designator to http://www.w3.org/TR/profile/sdp-us. Then
fix the example to match.