Use correct method in docs

[timneutkens]

I noticed this while writing a webpack plugin, tap can't return a modification, register can, so that's the correct method to show.

[codecov]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #68   +/-   ##
======================================
  Coverage    91.9%   91.9%           
======================================
  Files          15      15           
  Lines         519     519           
  Branches       71      71           
======================================
  Hits          477     477           
  Misses         39      39           
  Partials        3       3

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[TheLarkInn]

Thanks 🙇🙇