[Fix] Fix typo in FPN neck

[Xiangxu-0103]

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Modification

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[codecov]

Codecov Report

Merging #1405 (23aa1fe) into master (1b24ad6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1405   +/-   ##
=======================================
  Coverage   90.39%   90.39%           
=======================================
  Files         133      133           
  Lines        7906     7906           
  Branches     1318     1318           
=======================================
  Hits         7147     7147           
  Misses        536      536           
  Partials      223      223           

Flag	Coverage Δ
unittests	90.39% <ø> (ø)

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Impacted Files	Coverage Δ
mmseg/models/necks/fpn.py	57.69% <ø> (ø)

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