fix config bug in docs/tutorials/finetune.md (#3045)

`target_means` and `target_stds` should be in bbox_coder according to
https://github.com/open-mmlab/mmdetection/blob/2b6f6616f804beaca3dbf071fa398c586243db13/configs/_base_/models/mask_rcnn_r50_fpn.py#L48