Fix: PlainParser using fix in presentation (#9239)

### What problem does this PR solve?

tiny fix about the using of `deepdoc.pdf_parser.PlainParser` in
`rag.app.presentation.chunk`, I referred to other ways of using this
class.
So tiny the fix is, a issue seems unnecessary.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/app/presentation.py

             sections = pdf_parser(filename, binary, from_page=from_page, to_page=to_page, callback=callback)
         elif layout_recognizer == "Plain Text":
             pdf_parser = PlainParser()
-            sections, _ = pdf_parser(filename, binary, from_page=from_page, to_page=to_page, callback=callback)
+            sections, _ = pdf_parser(filename if not binary else binary, from_page=from_page, to_page=to_page,
+                                      callback=callback)
         else:
             vision_model = LLMBundle(kwargs["tenant_id"], LLMType.IMAGE2TEXT, llm_name=layout_recognizer, lang=lang)
             pdf_parser = VisionParser(vision_model=vision_model, **kwargs)