Fix link to Docs (#393)

Author: blink1073

README.md

@@ -1,7 +1,7 @@
 # jupyterlab server
 
 [![Build Status](https://github.com/jupyterlab/jupyterlab_server/workflows/Tests/badge.svg?branch=master)](https://github.com/jupyterlab/jupyterlab_server/actions?query=branch%3Amaster+workflow%3A%22Tests%22)
-[![Documentation Status](https://readthedocs.org/projects/jupyterlab_server/badge/?version=stable)](http://jupyterlab_server.readthedocs.io/en/stable/)
+[![Documentation Status](https://readthedocs.org/projects/jupyterlab-server/badge/?version=stable)](http://jupyterlab-server.readthedocs.io/en/stable/)
 
 ## Motivation