fix testsuite/tests/typing-warnings/coercions.ml.reference
    
    The change is benine: currently, error messages about format6 also
    give its definition as a product of an inner format and a string: the
    message changes, but the semantics is the same.
    
    Ultimately, we want the error message *not* to change (we don't want
    the internal implementation of formats to be exposed to the innocent
    user), and that will be achieved by converting format6 to a nominal
    type instead of a structural pair.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14846 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02