Fix "Try it" request serialization in Update Subscription API docs

The "Try it" button on the "Update subscription" API documentation page (https://docs.creem.io/api-reference/endpoint/update-subscription) is incorrectly serializing the request body. It adds an extra property "0" containing the item details, which causes the API to return a "400 Bad Request" with the message "property 0 should not exist". The documentation should be updated to ensure the "Try it" functionality generates a valid JSON body with the items array correctly formatted.