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)