message: doc: Fix example code using deprecated xrange
  xrange was abandoned and doesn't exist in Python 3. range() works just the same
diff: @@ -209,7 +209,7 @@ find the average error, we calculate the arithmetical mean of the errors calcula
  calibration images.
  @code{.py}
  mean_error = 0
  -for i in xrange(len(objpoints)):
  +for i in range(len(objpoints)):
  imgpoints2, _ = cv.projectPoints(objpoints[i], rvecs[i], tvecs[i], mtx, dist)
  error = cv.norm(imgpoints[i], imgpoints2, cv.NORM_L2)/len(imgpoints2)
  mean_error += error
  