mmdetection3d pdf file cover error

[JeffDing]

mmdetection doc page : https://mmdetection3d.readthedocs.io/zh_CN/latest/ , In this page i found pdf link, it return to the pdf download link: https://mmdetection3d.readthedocs.io/_/downloads/zh_CN/latest/pdf/

This pdf is mmdetection3d doc pdf file, but the pdf cover is mmcv

mmdetection en version also have this problem

[Tai-Wang]

Thanks for your reminder! Could you please try to fix this problem with a PR?