Fix link to babel setup doc from webpack doc

[NTARelix]

Summary
Found a broken link while browsing webpack setup page. This pull request fixes that link.

Test plan
If there's some way to resolve deep markdown links with the static site generator then broken links like this would never make it past the build step.

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[codecov-io]

Codecov Report

Merging #5355 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5355   +/-   ##
=======================================
  Coverage   61.29%   61.29%           
=======================================
  Files         205      205           
  Lines        6917     6917           
  Branches        4        3    -1     
=======================================
  Hits         4240     4240           
  Misses       2676     2676           
  Partials        1        1

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[cpojer]

Thanks!

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