Fix typo in API Key environment variable example

[168SDTH]

此处"$DEEPSEEK_API_KEY"更合理，原"$DEESEEK_API_KEY"疑似"Deepseek"漏写了一个"p"

Modifications / 改动点

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Documentation:

• Fix the DeepSeek API key environment variable name in the Chinese provider setup guide example.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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[gemini-code-assist]

Code Review

This pull request corrects a typo in the documentation for model provider configuration, specifically updating the example environment variable from $DEESEEK_API_KEY to $DEEPSEEK_API_KEY. I have no feedback to provide as there were no review comments and the change is a simple documentation fix.