Fix detect.py --update list bug (#8783)

Fix detect.py --update

Resolves https://github.com/ultralytics/yolov5/issues/8776

detect.py

@@ -210,7 +210,7 @@ def run(
         s = f"\n{len(list(save_dir.glob('labels/*.txt')))} labels saved to {save_dir / 'labels'}" if save_txt else ''
         LOGGER.info(f"Results saved to {colorstr('bold', save_dir)}{s}")
     if update:
-        strip_optimizer(weights)  # update model (to fix SourceChangeWarning)
+        strip_optimizer(weights[0])  # update model (to fix SourceChangeWarning)
 
 
 def parse_opt():