Update py_face_detection.rst

bug in example

doc/py_tutorials/py_objdetect/py_face_detection/py_face_detection.rst

@@ -73,7 +73,7 @@ Now we find the faces in the image. If faces are found, it returns the positions
 
     faces = face_cascade.detectMultiScale(gray, 1.3, 5)
     for (x,y,w,h) in faces:
-        img = cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)
+        cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)
         roi_gray = gray[y:y+h, x:x+w]
         roi_color = img[y:y+h, x:x+w]
         eyes = eye_cascade.detectMultiScale(roi_gray)