samples: fix python samples compatibility with demo.py

samples/python/lk_track.py

@@ -23,6 +23,7 @@ from __future__ import print_function
 
 import numpy as np
 import cv2 as cv
+
 import video
 from common import anorm2, draw_str
 from time import clock
@@ -96,9 +97,11 @@ def main():
     except:
         video_src = 0
 
-    print(__doc__)
     App(video_src).run()
-    cv.destroyAllWindows()
+    print('Done')
+
 
 if __name__ == '__main__':
+    print(__doc__)
     main()
+    cv.destroyAllWindows()