Update detect.py (#2167)

Without this cv2.imshow opens a window but nothing is visible

detect.py

@@ -118,6 +118,7 @@ def detect(save_img=False):
             # Stream results
             if view_img:
                 cv2.imshow(str(p), im0)
+                cv2.waitKey(1) # 1 millisecond
 
             # Save results (image with detections)
             if save_img: