Fix Mat indexing in TrackingMotion tutorial code (supersedes pull request #38)

samples/cpp/tutorial_code/TrackingMotion/cornerDetector_Demo.cpp

@@ -55,8 +55,8 @@ int main( int argc, char** argv )
   for( int j = 0; j < src_gray.rows; j++ )
      { for( int i = 0; i < src_gray.cols; i++ )
           {
-            float lambda_1 = myHarris_dst.at<float>( j, i, 0 );
-            float lambda_2 = myHarris_dst.at<float>( j, i, 1 );
+            float lambda_1 = myHarris_dst.at<Vec6f>(j, i)[0];
+            float lambda_2 = myHarris_dst.at<Vec6f>(j, i)[1];
             Mc.at<float>(j,i) = lambda_1*lambda_2 - 0.04*pow( ( lambda_1 + lambda_2 ), 2 );
           }
      }