message: Fix typo in documentation example
diff: @@ -149,7 +149,7 @@ object:
  res.version = 11; // HTTP/1.1
  res.status = 200;
  res.reason = "OK";
  - res.fields.insert("Sever", "Beast");
  + res.fields.insert("Server", "Beast");
  res.fields.insert("Content-Length", 4);
  res.body = "****";
  ```
  