docs(Usage): fix link to Semantic UI CSS

[layershifter]

Fixes #3232.

[codecov-io]

Codecov Report

Merging #3236 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3236   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         169      169           
  Lines        2790     2790           
=======================================
  Hits         2788     2788           
  Misses          2        2

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