Update usps.py

[youkaichao]

The backslash should be escaped. Currently, it looks bad:

[codecov-io]

Codecov Report

Merging #1117 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1117      +/-   ##
==========================================
- Coverage    64.9%   64.77%   -0.13%     
==========================================
  Files          68       68              
  Lines        5417     5417              
  Branches      835      835              
==========================================
- Hits         3516     3509       -7     
- Misses       1645     1649       +4     
- Partials      256      259       +3

Impacted Files	Coverage Δ
torchvision/datasets/usps.py	32.35% <ø> (ø)	⬆️
torchvision/transforms/transforms.py	80.35% <0%> (-1.18%)	⬇️
torchvision/transforms/functional.py	71.09% <0%> (-0.29%)	⬇️

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[fmassa]

Thanks!