docs: use fulfillment instead of shipping in order totals example

[misza-one]

Description

Fixes the order capability example to use "fulfillment" instead of "shipping" in the totals array. The Total schema defines the type enum as: items_discount, subtotal, discount, fulfillment, tax, fee, total. The value shipping is not valid for totals.

Single line change in docs/specification/order.md:

-    { "type": "shipping", "amount": 1200 },
+    { "type": "fulfillment", "amount": 1200 },

Note: Other "type": "shipping" occurrences in the codebase are in fulfillment.methods[].type context, which correctly references fulfillment_method.json (enum: shipping, pickup). Only this totals example was using the wrong value.

Fixes #63

Type of change

• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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