Fix incorrect toml_document variable name (#577)

Author: nathanjmcdougall

src/usethis/_integrations/file/toml/io_.py

@@ -277,8 +277,8 @@ def extend_list(self, *, keys: Sequence[Key], values: list[Any]) -> None:
             for key in reversed(keys):
                 contents = {key: contents}
             assert isinstance(contents, dict)
-            pyproject = mergedeep.merge(toml_document, contents)
-            assert isinstance(pyproject, TOMLDocument)
+            toml_document = mergedeep.merge(toml_document, contents)
+            assert isinstance(toml_document, TOMLDocument)
         else:
             TypeAdapter(dict).validate_python(p_parent)
             TypeAdapter(list).validate_python(d)