RM ANOVA: fixes issue forum "Multiple Covariates Error"

[raviselker]

No description provided.

[codecov-io]

Codecov Report

Merging #141 into master will decrease coverage by 0.01%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
- Coverage   67.68%   67.66%   -0.02%     
==========================================
  Files          42       42              
  Lines       12467    12470       +3     
==========================================
  Hits         8438     8438              
- Misses       4029     4032       +3

Impacted Files	Coverage Δ
R/anovarm.b.R	66.57% <40%> (-0.28%)	⬇️

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