doe: fix handling of more than one generator

[JTPetter]

• fixes [Bug]: 2 or more generators impossible  jasp-issues#4051

[github-actions]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 62 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.53%. Comparing base (8c33634) to head (9645c24).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
R/doeFactorial.R	0.00%	62 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
- Coverage   84.96%   84.53%   -0.44%     
==========================================
  Files          18       18              
  Lines       10012    10063      +51     
==========================================
  Hits         8507     8507              
- Misses       1505     1556      +51     

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