Fix RTD badge

README.md

@@ -2,7 +2,7 @@
 
 [![Join the chat at https://gitter.im/nodemcu/nodemcu-firmware](https://img.shields.io/gitter/room/badges/shields.svg)](https://gitter.im/nodemcu/nodemcu-firmware?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://travis-ci.org/nodemcu/nodemcu-firmware.svg)](https://travis-ci.org/nodemcu/nodemcu-firmware)
-[![Documentation Status](https://readthedocs.com/projects/nodemcu/badge/?version=dev)](http://nodemcu.readthedocs.io/)
+[![Documentation Status](https://img.shields.io/badge/docs-master-yellow.svg?style=flat)](http://nodemcu.readthedocs.io/)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg?style=flat)](https://github.com/nodemcu/nodemcu-firmware/blob/master/LICENSE)
 
 ### A Lua based firmware for ESP8266 WiFi SOC