DOC: Correct documentation to GroupBy.rank

[gfyoung]

Closes #20694.

cc @peterpanmj

[codecov]

Codecov Report

Merging #20708 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #20708      +/-   ##
==========================================
+ Coverage   91.81%   91.84%   +0.02%     
==========================================
  Files         153      153              
  Lines       49279    49279              
==========================================
+ Hits        45245    45259      +14     
+ Misses       4034     4020      -14

Flag	Coverage Δ
#multiple	90.23% <ø> (+0.02%)	⬆️
#single	41.9% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/groupby/groupby.py	92.55% <ø> (ø)	⬆️
pandas/core/frame.py	97.16% <0%> (ø)	⬆️
pandas/util/testing.py	84.59% <0%> (+0.2%)	⬆️
pandas/plotting/_converter.py	66.81% <0%> (+1.73%)	⬆️

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[jreback]

thanks @gfyoung