fix mgf of NegativeBinomial

[jw3126]

fix #1603

[codecov-commenter]

Codecov Report

Merging #1604 (ba68508) into master (506ec16) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1604      +/-   ##
==========================================
- Coverage   85.84%   85.84%   -0.01%     
==========================================
  Files         129      129              
  Lines        8018     8016       -2     
==========================================
- Hits         6883     6881       -2     
  Misses       1135     1135              

Impacted Files	Coverage Δ
src/univariate/discrete/geometric.jl	89.33% <100.00%> (-0.15%)	:arrow_down:
src/univariate/discrete/negativebinomial.jl	94.52% <100.00%> (-0.08%)	:arrow_down:

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