remove end double quotes and convert type to string (#1036)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6465]
Fix some minor style issue

## What changed?
<!-- Provide a bulleted list in the present tense -->
* remove unnecessary trailing quote
* ensure the value is string format

## Release notes draft
n/a

## Anything else?
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ping @bigcommerce/dev-docs @bigcommerce/team-orders 

[DEVDOCS-6465]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6465?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: bc-vincent-zhao

reference/orders.v2.oas2.yml

@@ -1966,7 +1966,7 @@ components:
                 staff_notes: BIN-45
                 customer_message: Custom Journal Added
                 discount_amount: '0'
-                coupon_discount": 0
+                coupon_discount: '0'
                 shipping_address_count: 1
                 ebay_order_id: '0'
                 cart_id: 8b84f622-faf1-4c10-887b-f5dff2f9eaf4