docs: 修正兼容模式升级说明中的中文错别字

[Freyiz]

What this PR does / why we need it?

修正 docs/fastjson_1_upgrade_cn.md 第 2.2 节「兼容模式升级」中的中文错别字，避免读者误解。

Summary of your change

将「认证测试」改为「请认真测试」。

原文：

通过这样的模式升级虽然省事，认证测试，遇到问题反馈到 ...

修改后：

通过这样的模式升级虽然省事，请认真测试，遇到问题反馈到 ...

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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