PROMO-1296 addressed PR comments

Author: bc-anshumishra

reference/promotions.v3.yml

@@ -339,10 +339,10 @@ paths:
                   example: _
                   maxLength: 1
                 exclude_characters:
-                  pattern: '[A-Z0-9]'
                   type: array
                   items:
                     type: string
+                    pattern: '[A-Z0-9]'
                   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