Minor update for Simplified Chinese Translation

[flytothehighest]

Update index.md: 1.Translate the missing part:

If you manage a web site, consider [adding the countdown banner](https://github.com/keepandroidopen/keepandroidopen.github.io/blob/main/banner) to the top of your page.

2.Modify some ambiguous words to ensure accuracy.

[marcprux]

Thanks!