updating readme binder link

this matches it with the one in the documentation (also updates the link badge)

README.md

@@ -13,7 +13,7 @@
 [![Documentation Status](https://readthedocs.org/projects/jupyterlab/badge/?version=stable)](http://jupyterlab.readthedocs.io/en/stable/)
 [![Google Group](https://img.shields.io/badge/-Google%20Group-lightgrey.svg)](https://groups.google.com/forum/#!forum/jupyter)
 [![Join the Gitter Chat](https://img.shields.io/gitter/room/nwjs/nw.js.svg)](https://gitter.im/jupyterlab/jupyterlab)
-[![Binder](https://mybinder.org/badge.svg)](https://mybinder.org/v2/gh/jupyterlab/jupyterlab-demo/0b0bb42e3e43ee2ebe1c0424d3a88a9b9edcd055?urlpath=lab%2Ftree%2Fdemo%2FLorenz.ipynb)
+[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/jupyterlab/jupyterlab-demo/master?urlpath=lab/tree/demo)
 
 An extensible environment for interactive and reproducible computing, based on the
 Jupyter Notebook and Architecture. [Currently ready for users.](https://blog.jupyter.org/jupyterlab-is-ready-for-users-5a6f039b8906)