openapi: Fix /settings/notifications OpenAPI documentation.

We pass these values encoded as JSON and parsed with
validator=check_int, not as raw integers.

zerver/openapi/zulip.yaml

@@ -3664,6 +3664,8 @@ paths:
           * 1 - All unreads
           * 2 - Private messages and mentions
           * 3 - None
+        content:
+          application/json:
             schema:
               type: integer
               enum: