🐛 Fix undefined variable references in write_file() (#510)

Fix bug introduced in PR #489 where write_file() was referencing
undefined variables (parse_stack/append_middleware) instead of the
renamed function parameters (unparse_stack/prepend_middleware).

Author: MiWeiss

bibtexparser/entrypoint.py

@@ -151,8 +151,8 @@ def write_file(
     :param bibtex_format: Customized BibTeX format to use (optional)."""
     bibtex_str = write_string(
         library=library,
-        unparse_stack=parse_stack,
-        prepend_middleware=append_middleware,
+        unparse_stack=unparse_stack,
+        prepend_middleware=prepend_middleware,
         bibtex_format=bibtex_format,
     )
     if isinstance(file, str):