Fix: markdown table conversion error (#7570)

### What problem does this PR solve?

Since `import markdown.markdown` has been changed to `import markdown`
in `rag/app/naive.py`, previous code for converting markdown tables
would call a markdown module instead of a callable function. This cause
error.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

rag/app/naive.py

@@ -22,7 +22,7 @@ from timeit import default_timer as timer
 
 from docx import Document
 from docx.image.exceptions import InvalidImageStreamError, UnexpectedEndOfFileError, UnrecognizedImageError
-import markdown 
+from markdown import markdown 
 from PIL import Image
 from tika import parser
 
@@ -298,8 +298,7 @@ class Markdown(MarkdownParser):
             return []
         
         from bs4 import BeautifulSoup
-        md = markdown.Markdown()
-        html_content = md.convert(text)
+        html_content = markdown(text)
         soup = BeautifulSoup(html_content, 'html.parser')
         html_images = [img.get('src') for img in soup.find_all('img') if img.get('src')]
         return html_images
@@ -344,7 +343,6 @@ class Markdown(MarkdownParser):
                     sections.append((sec_ + "\n" + sec, ""))
                 else:
                     sections.append((sec, ""))
-
         for table in tables:
             tbls.append(((None, markdown(table, extensions=['markdown.extensions.tables'])), ""))
         return sections, tbls