Correct version specification

Author: krassowski

setup.cfg

@@ -18,7 +18,7 @@ classifiers =
 project_urls =
     Bug Tracker = https://github.com/krassowski/docstring-to-markdown/issues
     Source Code = https://github.com/krassowski/docstring-to-markdown
-version = attr: package.__version__
+version = attr: docstring_to_markdown.__version__
 
 [options]
 packages = find: