Fix Git and GitHub Secrets talk link

[lonelyhty]

Summary

• Replace the broken Confreaks Git and GitHub Secrets talk link with Zach Holman's current talk page.
• Update the same link in the Korean, Simplified Chinese, and Traditional Chinese READMEs.

Context

Fixes #225.

Validation

• http://confreaks.tv/videos/aloharuby2012-git-and-github-secrets now redirects to an unrelated page.
• http://www.confreaks.com/videos/1229-aloharuby2012-git-and-github-secrets returns 404.
• https://zachholman.com/talk/git-github-secrets/ returns HTTP 200.
• Ran git diff --check.