Change binder link to a jlab 3 prebuilt image.

See https://github.com/jupyterlab/jupyterlab-demo/pull/102

docs/source/index.rst

@@ -7,7 +7,7 @@
 JupyterLab Documentation
 ########################
 
-JupyterLab is the next-generation web-based user interface for Project Jupyter. `Try it on Binder <https://mybinder.org/v2/gh/jupyterlab/jupyterlab-demo/master?urlpath=lab/tree/demo>`__.  JupyterLab follows the Jupyter `Community Guides <https://jupyter.readthedocs.io/en/latest/community/content-community.html>`__.
+JupyterLab is the next-generation web-based user interface for Project Jupyter. `Try it on Binder <https://mybinder.org/v2/gh/jupyterlab/jupyterlab-demo/8f2ecb6?urlpath=lab/tree/demo>`__.  JupyterLab follows the Jupyter `Community Guides <https://jupyter.readthedocs.io/en/latest/community/content-community.html>`__.
 
 .. image:: ./images/jupyterlab.png
    :align: center