Added nonfree header in "Feature description" tutorial code sample (bug #2527)

doc/tutorials/features2d/feature_description/feature_description.rst

@@ -32,6 +32,7 @@ This tutorial code's is shown lines below. You can also download it from `here <
    #include "opencv2/core/core.hpp"
    #include "opencv2/features2d/features2d.hpp"
    #include "opencv2/highgui/highgui.hpp"
+   #include "opencv2/nonfree/features2d.hpp"
 
    using namespace cv;