Use rowwiseDT in vignette construction

[MichaelChirico]

I think it's a fair amount clearer in this case. Probably it could be applied in other vignettes too, but sticking with the small change for now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (e1409e8) to head (234b8f6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7083   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          79       79           
  Lines       14677    14677           
=======================================
  Hits        14486    14486           
  Misses        191      191           

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