Fix erroneous documentation of the syntax of CREATE CONSTRAINT TRIGGER.

Fix erroneous documentation of the syntax of CREATE CONSTRAINT TRIGGER.

The grammar requires a specific ordering of the clauses, but the
documentation showed a different order.  This error was introduced in
commit b47953f9c69d48a9261bd643e3170017b93f6337, which merged the CREATE
CONSTRAINT TRIGGER documentation into the CREATE TRIGGER page.  There is
no code bug AFAICS.