PROMO-1296 Added required sibling items field for type:array

Author: bc-anshumishra

reference/promotions.v3.yml

@@ -341,6 +341,8 @@ paths:
                 exclude_characters:
                   pattern: '[A-Z0-9]'
                   type: array
+                  items:
+                    type: string
                   description: 'An optional array of single length string elements to exclude from the generated coupon codes. Only capital letters and numbers are allowed. The default value is an empty array [].'
                   example: ["1", "2", "3", "4", "5", "A", "B", "C", "D", "E"]
                   maxLength: 26