hotfix: call isOpened() in python bg sub tutorial

opencv#19299

Author: alkasm

samples/python/tutorial_code/video/background_subtraction/bg_sub.py

@@ -18,7 +18,7 @@
 
 ## [capture]
 capture = cv.VideoCapture(cv.samples.findFileOrKeep(args.input))
-if not capture.isOpened:
+if not capture.isOpened():
     print('Unable to open: ' + args.input)
     exit(0)
 ## [capture]