Use tljh.jupyter.org/bootstrap.py to get installer

- Shorter URL makes the horizontal scrollbar in the TLJH
  docs site much better!
- Shorter URLs are cooler
- This redirect is managed by readthedocs, with the interface
  available at https://readthedocs.org/dashboard/the-littlest-jupyterhub/redirects/

docs/install/google.rst

@@ -144,7 +144,7 @@ Let's create the server on which we can run JupyterHub.
    .. code-block:: bash
 
        #!/bin/bash
-       curl https://raw.githubusercontent.com/jupyterhub/the-littlest-jupyterhub/master/bootstrap/bootstrap.py \
+       curl https://tljh.jupyter.org/bootstrap.py \
          | sudo python3 - \
            --admin <admin-user-name>