fixed typo (#861)
fixed typo (#861)

Summary: Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/861

Differential Revision: D19879528

Pulled By: ppwwyyxx

fbshipit-source-id: 3b4baa40a5157da52986d07011cec7216ea224c5

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2