Fix the counting error in burndown

[benmatselby]

No description provided.

[codecov]

Codecov Report

Merging #5 into master will increase coverage by 1.44%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   75.36%   76.81%   +1.44%     
==========================================
  Files           4        4              
  Lines         138      138              
==========================================
+ Hits          104      106       +2     
+ Misses         30       29       -1     
+ Partials        4        3       -1

Impacted Files	Coverage Δ
cmd/burndown.go	91.3% <100%> (+4.34%)	⬆️

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