docs: update fulfillment and total amounts in discount spec example

[kev-mao]

The mixed-discounts response example in the discount extension shows a 599 free-shipping discount as both discount: -599 and fulfillment: 0, with total: 2601. This double-counts the discount. This change keeps the shipping discount in the discount line (per Impact on Line Items and Totals) and fixes the example to show the gross fulfillment: 599 and total: 3200, satisfying the totals summation invariant.

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• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
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