Fix deadlink to Fairlearn docs (#344)

Author: kmaehashi

README.md

@@ -15,7 +15,7 @@ Sites that are using this theme:
 - Bokeh: https://docs.bokeh.org/en/dev/
 - JupyterHub and Binder: https://docs.mybinder.org/, http://z2jh.jupyter.org/en/latest/, https://repo2docker.readthedocs.io/en/latest/, https://jupyterhub-team-compass.readthedocs.io/en/latest/
 - Jupyter Book beta version uses an extension of this theme: https://beta.jupyterbook.org
-- Fairlearn: https://fairlearn.github.io/master/quickstart.html
+- Fairlearn: https://fairlearn.github.io/main/quickstart.html
 
 
 ## Installation and usage