message: Fix code sample in websocket.qbk
diff: @@ -226,7 +226,7 @@ void echo(beast::websocket::stream<boost::asio::ip::tcp::socket>& ws)
  {
  beast::streambuf sb;
  beast::websocket::opcode::value op;
  - ws.read(sb);
  + ws.read(op, sb);
  ws.set_option(beast::websocket::message_type{op});
  ws.write(sb.data());
  