Fix:Unimplemented function in ppt_parser (#8095)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/8088

### Type of change

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

deepdoc/parser/ppt_parser.py

@@ -63,7 +63,7 @@ class RAGFlowPptParser:
             if shape_type == 6:
                 texts = []
                 for p in sorted(shape.shapes, key=lambda x: (x.top // 10, x.left)):
-                    t = self.__extract_texts(p)
+                    t = self.__extract(p)
                     if t:
                         texts.append(t)
                 return "\n".join(texts)