Fix CSS paths in react-menu documentation.

[PelicanMaster]

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[PelicanMaster]

@googlebot I signed it!

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[codecov-io]

Codecov Report

Merging #998 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #998   +/-   ##
=======================================
  Coverage   94.13%   94.13%           
=======================================
  Files          86       86           
  Lines        3649     3649           
  Branches      578      578           
=======================================
  Hits         3435     3435           
  Misses         90       90           
  Partials      124      124

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[OZhurbenko]

Just had this exact problem, let's merge this, maybe?