fix: Chinese README 'English' link points to index.html instead of README.html

pkgdown's build_home() renders the English README as index.html,
not README.html. The link on README_ZH.html must point to index.html.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: zorrooz

README_ZH.md

@@ -7,7 +7,7 @@
 [![lint](https://github.com/zorrooz/plotit/actions/workflows/lint.yaml/badge.svg)](https://github.com/zorrooz/plotit/actions/workflows/lint.yaml)
 <!-- badges: end -->
 
-<p align="center"><b>简体中文</b> | <a href="README.html">English</a></p>
+<p align="center"><b>简体中文</b> | <a href="index.html">English</a></p>
 
 > ⚠️ **早期开发阶段**  
 > plotit 处于活跃的预发布开发中。每次更新都**极有可能**带来破坏性变更。