Update index.en.md

[ouba421]

添加validateFields 第二个参数的英文文档注释

[github-actions[bot]]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.72%. Comparing base (495be63) to head (1d1244c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6785 +/- ## ======================================= Coverage 92.72% 92.72% ======================================= Files 335 335 Lines 7198 7198 Branches 1804 1767 -37 ======================================= Hits 6674 6674 - Misses 489 516 +27 + Partials 35 8 -27 ```

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[zombieJ]

合并到 https://github.com/ant-design/ant-design-mobile/pull/6783 里一起？