fixed display_image tutorial sample

samples/cpp/tutorial_code/introduction/display_image/display_image.cpp

@@ -1,21 +1,23 @@
 #include <opencv2/core/core.hpp>
 #include <opencv2/imgcodecs.hpp>
 #include <opencv2/highgui/highgui.hpp>
+
 #include <iostream>
+#include <string>
 
 using namespace cv;
 using namespace std;
 
 int main( int argc, char** argv )
 {
-    if( argc != 2)
+    string imageName("../data/HappyFish.jpg"); // by default
+    if( argc > 1)
     {
-     cout <<" Usage: display_image ImageToLoadAndDisplay" << endl;
-     return -1;
+        imageName = argv[1];
     }
 
     Mat image;
-    image = imread(argv[1], IMREAD_COLOR); // Read the file
+    image = imread(imageName.c_str(), IMREAD_COLOR); // Read the file
 
     if( image.empty() )                      // Check for invalid input
     {