fix: resolve cargo clippy warnings

[greenhandatsjtu]

there are 3 errors when running cargo clippy -- -D warnings , I fixed it

[casbin-bot]

@smrpn @hackerchai @PsiACE @GopherJ please review

[codecov]

Codecov Report

Merging #296 (90eea76) into master (4a661f3) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
- Coverage   81.52%   81.51%   -0.01%     
==========================================
  Files          23       23              
  Lines        3480     3479       -1     
==========================================
- Hits         2837     2836       -1     
  Misses        643      643              

Impacted Files	Coverage Δ
src/effector.rs	85.10% <ø> (-0.32%)	⬇️
src/rbac_api.rs	91.20% <0.00%> (ø)

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[PsiACE]

LGTM