Fix typo in main.rs

[Hayashi-Yudai]

Hello, I'm Hayashi-Yudai.

I use your repository and learn from the codes. This time, I found very tiny typo in help message. I'm grad if you merge my pull request.

Sincerely.

[codecov-io]

Codecov Report

Merging #38 into master will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   73.62%   73.41%   -0.22%     
==========================================
  Files           8        8              
  Lines         345      346       +1     
==========================================
  Hits          254      254              
- Misses         91       92       +1

[Hayashi-Yudai]

Remove return; to pass CI

[dmerejkowsky]

Thanks :)