docs: update interface in README (#50)

Signed-off-by: Michele Dolfi <[email protected]>

README.md

@@ -52,7 +52,7 @@ from docling.document_converter import DocumentConverter
 source = "https://arxiv.org/pdf/2206.01062"  # PDF path or URL
 converter = DocumentConverter()
 doc = converter.convert_single(source)
-print(doc.export_to_markdown())  # output: "## DocLayNet: A Large Human-Annotated Dataset for Document-Layout Analysis [...]"
+print(doc.render_as_markdown())  # output: "## DocLayNet: A Large Human-Annotated Dataset for Document-Layout Analysis [...]"
 ```
 
 ### Convert a batch of documents