ModelTree: fix call to ExprNode::computeTemporaryTerms()

The first argument to ExprNode::computeTemporaryTerms() is supposed to be a
pair (endo derivation order, param derivation order). The two elements were
interverted in the call. This would not affect the result, because parameter
derivatives are not computed there and what matters is the lexical ordering
which remains the same. But fixing the order is better, for consistency with
the method description.