Correct free usage statement of serper

[GithuBarry]

📝 信息 | Information

Fix descriptions of Serper free usage -- free credit does not reset monthly.

✅ 检查列表 | Checklist:

• [x] 我已阅读了 Readme.md
• [x] 描述是用英文编写的。
• [x] meta.json 和 plugin-template.json 没有被修改过。
• [x] entry 放置在 src 目录中，并且使用 .json 文件扩展名。

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• [x] I have read the Readme.md
• [x] The description is written in English.
• [x] The meta.json and plugin-template.json have not been modified.
• [x] The entry is placed in the src directory with the .json file extension.

[lobehubbot]

❤️ Great PR @GithuBarry ❤️

The growth of project is inseparable from user feedback and contribution, thanks for your contribution!

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