Incorrect citation in transformer documentation.

[whughw]

The original citation of the paper On Layer Normalization in the Transformer Architecture https://arxiv.org/abs/2007.08103 is wrong.
It should be https://arxiv.org/abs/2002.04745

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Codecov Report

Merging #974 (3c7daed) into master (a764163) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #974   +/-   ##
=======================================
  Coverage   65.73%   65.73%           
=======================================
  Files         150      150           
  Lines        9517     9517           
  Branches     1726     1726           
=======================================
  Hits         6256     6256           
  Misses       2938     2938           
  Partials      323      323           

Flag	Coverage Δ
unittests	65.73% <ø> (ø)

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Impacted Files	Coverage Δ
mmcv/cnn/bricks/transformer.py	0.00% <ø> (ø)

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[jshilong]

Thanks for your correction.

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@whughw
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