fixed incorrect code in introduction tutorial

doc/tutorials/introduction/linux_gcc_cmake/linux_gcc_cmake.rst

@@ -30,19 +30,24 @@ Let's use a simple program such as DisplayImage.cpp shown below.
 
   using namespace cv;
 
-   int main( int argc, char** argv )
+  int main(int argc, char** argv )
   {
+      if ( argc != 2 )
+      {
+          printf("usage: DisplayImage.out <Image_Path>\n");
+          return -1;
+      }
+
       Mat image;
       image = imread( argv[1], 1 );
 
-     if( argc != 2 || !image.data )
+      if ( !image.data )
       {
-         printf( "No image data \n" );
+          printf("No image data \n");
           return -1;
       }
-
-     namedWindow( "Display Image", CV_WINDOW_AUTOSIZE );
-     imshow( "Display Image", image );
+      namedWindow("Display Image", CV_WINDOW_AUTOSIZE );
+      imshow("Display Image", image);
 
       waitKey(0);