Confused in choosing constant reference or constant value to avoid post modification on variables in C++

I am relatively new to C++. When learning OpenCV I need to name a window. The title should not be changed afterwards, so I should make it constant. There are two options:

const std::string title = "testing...";

or

const std::string& title = "testing...";

Both do not allow us to modify the declared or defined variables.

Could you tell me which one should I choose?

#include <iostream>

#include <opencv2/opencv.hpp>

int main(int, char **)
{
    cv::Mat image = cv::imread("../test.png");
    const std::string title = "testing...";
    // const std::string& title = "testing...";
    cv::imshow(title, image);
    cv::waitKey();
    return 0;
}