Update single mailing list return to be an array

This change is to match the legacy endpoint, which wraps the result into an array. While not ideal, the original goal is to in place replace the API for now, before deprecating/changing APIs.

spec/openapi.yaml

@@ -73,7 +73,7 @@ paths:
                content:
                   application/json:
                      schema:
-                        $ref: "#/components/schemas/MailingList"
+                        $ref: "#/components/schemas/MailingLists"
             404:
                description: Not found
             500: