Show Doc Badge of stable version

Author: fsschneider

README.md

@@ -3,7 +3,7 @@
 ![DeepOBS](docs/deepobs_banner.png "DeepOBS")
 
 [![PyPI version](https://badge.fury.io/py/deepobs.svg)](https://badge.fury.io/py/deepobs)
-[![Documentation Status](https://readthedocs.org/projects/deepobs/badge/?version=latest)](https://deepobs.readthedocs.io/en/latest/?badge=latest)
+[![Documentation Status](https://readthedocs.org/projects/deepobs/badge/?version=stable)](https://deepobs.readthedocs.io/en/latest/?badge=stable)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)