Omit ultralytics/utils/callbacks from coverage (#4345)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/models/yolov8.md

@@ -101,7 +101,7 @@ You can use YOLOv8 for object detection tasks using the Ultralytics pip package.
         model.info()
 
         # Train the model on the COCO8 example dataset for 100 epochs
-        results model.train(data='coco8.yaml', epochs=100, imgsz=640)
+        results = model.train(data='coco8.yaml', epochs=100, imgsz=640)
 
         # Run inference with the YOLOv8n model on the 'bus.jpg' image
         results = model('path/to/bus.jpg')