Update default.yaml `imgsz` comment (#3551)

ultralytics/yolo/cfg/default.yaml

@@ -10,7 +10,7 @@ data:  # (str, optional) path to data file, i.e. coco128.yaml
 epochs: 100  # (int) number of epochs to train for
 patience: 50  # (int) epochs to wait for no observable improvement for early stopping of training
 batch: 16  # (int) number of images per batch (-1 for AutoBatch)
-imgsz: 640  # (int) size of input images as integer or w,h
+imgsz: 640  # (int | list) input images size as int for train and val modes, or list[w,h] for predict and export modes
 save: True  # (bool) save train checkpoints and predict results
 save_period: -1 # (int) Save checkpoint every x epochs (disabled if < 1)
 cache: False  # (bool) True/ram, disk or False. Use cache for data loading