fix INSTALL.md
fix INSTALL.md

Summary: Pull Request resolved: https://github.com/fairinternal/detectron2/pull/441

Reviewed By: alexander-kirillov

Differential Revision: D22992850

Pulled By: ppwwyyxx

fbshipit-source-id: 222f1f78fdf756543eb3db9c2bf961625879458a

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2