Update README-cn.md
Update README-cn.md

修复几个小问题
- Mac的键盘修饰键`Option`不是`Alt`
- 命令应该使用代码块，便于阅读
- beta被写成了beat
drduh/OS-X-Yosemite-Security-and-Privacy-Guide,ScheerMT/OS-X-Yosemite-Security-and-Privacy-Guide,drduh/OS-X-Security-and-Privacy-Guide,drduh/macOS-Security-and-Privacy-Guide