DOC: Update ASV benchmark example in contributing.rst

[mroeschke]

Updating the ASV example based on the cleaning that was done in asv_bench/benchmarks/groupby.py

[codecov]

Codecov Report

Merging #19201 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19201      +/-   ##
==========================================
+ Coverage   91.51%   91.52%   +<.01%     
==========================================
  Files         147      147              
  Lines       48773    48773              
==========================================
+ Hits        44637    44639       +2     
+ Misses       4136     4134       -2

Flag	Coverage Δ
#multiple	89.89% <ø> (ø)	⬆️
#single	41.62% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	84.41% <0%> (+0.21%)	⬆️

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[jorisvandenbossche]

Thanks!