Fix code block formatting issue in Chapter 3, Section 2

[hqhq1025]

This pull request fixes a Markdown formatting issue in Chapter 3, Section 2 of the Hugging Face course.

• The Python code block was not properly displayed due to incorrect Markdown syntax.
• The code block is now correctly formatted and renders properly in the documentation.

Thank you for maintaining this excellent course!

本 PR 修复了 Hugging Face 课程第 3 章第 2 节中的 Markdown 格式错误。

• 原文中的 Python 代码块由于 Markdown 语法错误，无法正常显示。
• 现已修复，代码块可以正确显示，文档排版更清晰。

感谢您维护这么优秀的课程！

[HuggingFaceDocBuilderDev]

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