Fix Nim 2.0.0 compile error caused by `_` variables

[fukusuket]

Hello, Thank you so much for maintaining tool :) I created a fix PR for the issue below.

• Fix #105

In Nim 2.0.0 released on August 1st, it seems that _ can no longer be used as a variable due to the following changes.

• https://github.com/nim-lang/Nim/pull/21584
• https://github.com/nim-lang/Nim/issues/20687
• https://github.com/nim-lang/Nim/issues/21435

Currently, only the macOS code had _ usage, so I replaced it with x.

For this pull request, I have confirmed following:

• Successful compilation with Nim 2.0.0 and 1.6.14
• README some sample code works

I would appreciate it if you could review.

Regards,

[treeform]

Thanks this is great! Got to get 2.0 ready!

[fukusuket]

Thank you so much for quick review :)