improve a citation

[ThomasBreuer]

suggested by @fieker

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.50%. Comparing base (ac84098) to head (3642548).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4736      +/-   ##
==========================================
- Coverage   84.51%   84.50%   -0.01%     
==========================================
  Files         673      673              
  Lines       90135    90135              
==========================================
- Hits        76182    76173       -9     
- Misses      13953    13962       +9     

Files with missing lines	Coverage Δ
src/Rings/AbelianClosure.jl	94.36% <ø> (ø)

... and 3 files with indirect coverage changes

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