Fix typo in build_conv_layer docstring

[innerlee]

No description provided.

[codecov]

Codecov Report

Merging #928 (5e7b70b) into master (83cf25b) will not change coverage.
The diff coverage is n/a.

❗ Current head 5e7b70b differs from pull request most recent head f4b1558. Consider uploading reports for the commit f4b1558 to get more accurate results

@@           Coverage Diff           @@
##           master     #928   +/-   ##
=======================================
  Coverage   64.62%   64.62%           
=======================================
  Files         148      148           
  Lines        9354     9354           
  Branches     1698     1698           
=======================================
  Hits         6045     6045           
  Misses       2962     2962           
  Partials      347      347           

Flag	Coverage Δ
unittests	64.62% <ø> (ø)

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Impacted Files	Coverage Δ
mmcv/cnn/bricks/conv.py	100.00% <ø> (ø)

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