Update magic_pdf_parse_main.py

[samqin123]

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Motivation

以下代码是可以对目录中pdf文件进行循环处理，方便使用。修改前，由于文件打开后没有关闭，造成内存报错，感谢i12345的帮助，修改了代码，提交PR，请作者完善，造福大家。
Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

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Checklist

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• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
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• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

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