Update access_code type to string in opensign.yaml

Changed access_code type from number to string.

Author: raktima-opensignlabs

docs/docs/API-docs/v1.2/opensign.yaml

@@ -6427,7 +6427,7 @@ components:
             - approver
           example: signer
         access_code:
-          type: number
+          type: string
           format: number
           description: "Optional numeric access code required for the signer to access and sign the document."
           example: 123456