Fix Mask R-CNN doc

[ekagra-ranjan]

Fixes #1047.

[fmassa]

Thanks!

Just waiting for CI to finish

[codecov-io]

Codecov Report

Merging #1089 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1089      +/-   ##
==========================================
- Coverage   64.67%   64.52%   -0.15%     
==========================================
  Files          68       68              
  Lines        5407     5407              
  Branches      829      829              
==========================================
- Hits         3497     3489       -8     
- Misses       1661     1666       +5     
- Partials      249      252       +3

Impacted Files	Coverage Δ
torchvision/models/detection/mask_rcnn.py	81.35% <ø> (ø)	⬆️
torchvision/transforms/functional.py	69.94% <0%> (-1.45%)	⬇️
torchvision/transforms/transforms.py	81.18% <0%> (-0.6%)	⬇️
torchvision/models/vgg.py	89.55% <0%> (ø)	⬆️
torchvision/models/resnet.py	88.27% <0%> (ø)	⬆️

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