Fix mistake in config example.

config-example.json

@@ -168,5 +168,5 @@
   "session_secret": "233",
   "judge_server_addr": "http://127.0.0.1:5284",
   "judge_token": "233",
-  "email_jwt_token": "test"
+  "email_jwt_secret": "test"
 }