Update app.py

app.py

@@ -13,7 +13,7 @@ st.info("Use your webcam to interact with the virtual keyboard via hand gestures
 
 detector = HandDetector(maxHands=1, detectionCon=0.8)
 
-def video_frame_callback(frame):
+def video_frame_callback(frame: av.VideoFrame) -> av.VideoFrame:
     img = frame.to_ndarray(format="bgr24")
     hands, img = detector.findHands(img, flipType=False)