[bsp][n32] fix board image path in n32h760zil7-stb readme

[xuh0923]

Why to submit this PR

The BSP document for n32h760zil7-stb references a board image path that does not exist in the repository.
As a result, the board picture cannot be rendered when browsing the README.

What is your solution

• Update the image path in bsp/n32/n32hxxx/n32h760zil7-stb/README_zh.md
• Replace figures/board.png with the actual existing file figures/board.jpg

Provide the config and bsp

• BSP: bsp/n32/n32hxxx/n32h760zil7-stb
• .config: N/A (documentation-only change)
• action: N/A (documentation-only change)

Verification

• Verified that figures/board.jpg exists in the BSP directory
• Verified that figures/board.png does not exist
• Checked that the diff only updates the README image path

[CLAassistant]

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[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
To ensure your code complies with RT-Thread's coding style, please run the code formatting workflow by following the steps below (If the formatting of CI fails to run).

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🛠 操作步骤 | Steps

1. 前往 Actions 页面 | Go to the Actions page
   点击进入工作流 → | Click to open workflow →

2. 点击 Run workflow | Click Run workflow

• 设置需排除的文件/目录（目录请以"/"结尾）
  Set files/directories to exclude (directories should end with "/")
• 将目标分支设置为 \ Set the target branch to：docs-fix-n32h760-readme-image
• 设置PR number为 \ Set the PR number to：11526

3. 等待工作流完成 | Wait for the workflow to complete
   格式化后的代码将自动推送至你的分支。
   The formatted code will be automatically pushed to your branch.

完成后，提交将自动更新至 docs-fix-n32h760-readme-image 分支，关联的 Pull Request 也会同步更新。
Once completed, commits will be pushed to the docs-fix-n32h760-readme-image branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!