Update docstring for GIoU metric

[briankosw]

What does this PR do?

Updates the docstring for GIoU metric #469 (comment). @akihironitta

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[codecov]

Codecov Report

Merging #474 (8089344) into master (a72766c) will increase coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
+ Coverage   80.52%   80.75%   +0.22%     
==========================================
  Files         103      103              
  Lines        5710     5710              
==========================================
+ Hits         4598     4611      +13     
+ Misses       1112     1099      -13     

Flag	Coverage Δ
cpu	25.36% <ø> (ø)
pytest	25.36% <ø> (ø)
unittests	80.31% <ø> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pl_bolts/losses/object_detection.py	100.00% <ø> (ø)
pl_bolts/metrics/object_detection.py	100.00% <ø> (ø)
pl_bolts/datasets/cifar10_dataset.py	78.26% <0.00%> (+6.52%)	⬆️
pl_bolts/datasets/base_dataset.py	97.72% <0.00%> (+15.90%)	⬆️

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[akihironitta]

@briankosw Thank you! Can we have the same update for pytorch-lightning-bolts/pl_bolts/losses/object_detection.py?

[akihironitta]

@briankosw Looks great to me. Thank you for your contribution as always!