Add missing disambiguation to non-primary mapper example

Add missing disambiguation to non-primary mapper example

The mapping to B over a join defines an alternate primary key
based on all the primary key columns in the join unless we
re-define it explicitly.   Similarly, people expect that
``.id`` looks the same.  make sure these line up with the
old mapping.

Change-Id: I1ab064c57019e79c34293f6588d1e033f7083974