Caption missed in Fig.7

[HannahJHan]

Thanks for your meaningful work firstly!

A little typo. There is a single arrow without caption between 3DBN and PointPillars in the bottom part of figure 7 "Chronological overview of the most relevant deep learning-based 3D object detection methods". It seems to be MVX-Net inferring from other documents.

Looking forward to your next version！