Fix binder link

Author: xgarrido

README.md

@@ -69,7 +69,7 @@ To update your copy of the tutorial materials, navigate in a terminal to folder
 
 Click the badge below to run the tutorial online:
 
-[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/jupyter-widgets/tutorial/master?urlpath=%2Flab%2Ftree%2Fnotebooks)
+[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/jupyter-widgets/tutorial/main?urlpath=%2Flab%2Ftree%2Fnotebooks)
 
 Many of the materials work without modification on mybinder.org without needing to install anything on your computer. However, this is *not* the recommended way to do the tutorial.