update badge in readme

README.md

@@ -2,7 +2,7 @@
 
 [![PyPI version](https://badge.fury.io/py/big-fish.svg)](https://badge.fury.io/py/big-fish)
 [![Build Status](https://travis-ci.com/fish-quant/big-fish.svg?branch=master)](https://travis-ci.com/fish-quant/big-fish)
-[![Documentation Status](https://readthedocs.org/projects/big-fish/badge/?version=latest)](https://big-fish.readthedocs.io/en/latest/?badge=latest)
+[![Documentation Status](https://readthedocs.org/projects/big-fish/badge/?version=stable)](https://big-fish.readthedocs.io/en/latest/?badge=stable)
 [![codecov](https://codecov.io/gh/fish-quant/big-fish/branch/master/graph/badge.svg)](https://codecov.io/gh/fish-quant/big-fish)
 [![License](https://img.shields.io/badge/license-BSD%203--Clause-green)](https://github.com/fish-quant/big-fish/blob/master/LICENSE)
 [![Python 3.6](https://img.shields.io/badge/python-3.6-blue.svg)](https://www.python.org/downloads/release/python-360/)