fix doc build
fix doc build

Summary: follow up of D31575602 (https://github.com/facebookresearch/detectron2/commit/0e29b7ab8b77860b397aa02c2a164e1a8edcea8b)

Differential Revision: D31733407

fbshipit-source-id: f367fbcfd6764278bc263982159dac531e15c670

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2