Refactor parser code (#9042)

### What problem does this PR solve?

Refactor code

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

deepdoc/parser/html_parser.py

@@ -41,7 +41,7 @@ class RAGFlowHtmlParser:
     @classmethod
     def parser_txt(cls, txt):
         if not isinstance(txt, str):
-            raise TypeError("txt type should be str!")
+            raise TypeError("txt type should be string!")
         html_doc = readability.Document(txt)
         title = html_doc.title()
         content = html_text.extract_text(html_doc.summary(html_partial=True))