fix link to custom.css in documentation

[jugmac00]

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[codecov]

Codecov Report

Merging #2185 (f1b9d55) into rewrite (78b7772) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           rewrite    #2185   +/-   ##
========================================
  Coverage    99.85%   99.85%           
========================================
  Files          157      157           
  Lines         8944     8944           
  Branches       939      939           
========================================
  Hits          8931     8931           
  Misses           3        3           
  Partials        10       10           

Flag	Coverage Δ
tests	99.85% <ø> (ø)

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Impacted Files	Coverage Δ
src/tox/pytest.py	100.00% <ø> (ø)

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[gaborbernat]