graph/path: fix YenKShortestPaths

[kortschak]

Please take a look.

Fixes #1700.

/cc @liucan002

The corresponding graph rendered is this.

[codecov-commenter]

Codecov Report

Merging #1701 (750ad13) into master (054da3c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1701   +/-   ##
=======================================
  Coverage   73.26%   73.26%           
=======================================
  Files         510      510           
  Lines       60132    60132           
=======================================
+ Hits        44056    44058    +2     
+ Misses      13584    13583    -1     
+ Partials     2492     2491    -1     

Impacted Files	Coverage Δ
graph/path/yen_ksp.go	90.47% <ø> (+3.17%)	⬆️

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[sbinet]

the wikipedia link indeed reads for i from 0 to size(A[k − 1]) − 2 (and not for i from 0 to size(A[k − 1]) − 1) :)