Fix Chinese translation issues in chapters 1–3

[Neptune1729]

This PR consolidates several fixes to the Chinese translation in chapters 1–3.

chapter1/4.mdx

• Fixed a duplicated character typo in the explanation paragraph to improve readability.

chapter2/2.mdx

• Corrected the section title (use “分词器（tokenizer）” instead of the duplicated “tokenizer（ tokenizer ）”)
• Fixed the Markdown list structure by using proper nested bullet points
• Standardized the formatting of special tokens: [CLS], [SEP]

chapter3/2.mdx

• Removed a duplicated explanatory sentence in the tokenize_function example to avoid redundancy.

These changes improve clarity, accuracy, and MDX rendering consistency for the Chinese course content.

[HuggingFaceDocBuilderDev]

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