[Docs] Update README.md of PETR#2555

JingweiZhang12 merged 1 commit intoopen-mmlab:dev-1.xopen-mmlab/mmdetection3d:dev-1.xfrom

JingweiZhang12:petr_linkJingweiZhang12/mmdetection3d:petr_linkCopy head branch name to clipboard