docs: add 401 to possible status codes of /auth

Signed-off-by: Hsing-Yu (David) Chen <davidhsingyuchen@gmail.com>

api/swagger.yaml

@@ -8726,6 +8726,10 @@ paths:
               IdentityToken: "9cbaf023786cd7..."
         204:
           description: "No error"
+        401:
+          description: "Auth error"
+          schema:
+            $ref: "#/definitions/ErrorResponse"
         500:
           description: "Server error"
           schema: