Fix golint

[zhouzhuojie]

Fix golint report from https://goreportcard.com/report/github.com/checkr/flagr

[codecov-io]

Codecov Report

Merging #206 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   86.53%   86.66%   +0.13%     
==========================================
  Files          23       23              
  Lines        1329     1327       -2     
==========================================
  Hits         1150     1150              
+ Misses        129      128       -1     
+ Partials       50       49       -1

Impacted Files	Coverage Δ
pkg/entity/flag.go	94.28% <100%> (+5.09%)	⬆️

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