readme: Fix relative URL to demo_zbigarray.py

In the time when README.rst was initially introduced (58279ac7 "readme:
Initial draft") GitLab was not handling relative links well and it was
necessary to construct them properly by hand.

Nowdays GitLab automatically adds namespace and project and blob/ref/
prefix to relative URL link, so there is no need to specify them
manually.

Actually specifying the prefix manually makes URL invalid, as it becomes
something like

    https://lab.nexedi.com/nexedi/wendelin.core/blob/master/wendelin.core/blob/master/demo/demo_zbigarray.py

Fix it.

README.rst

@@ -69,5 +69,4 @@ Please see `demo/demo_zbigarray.py`__ for a complete example.
 .. _NEO:            http://www.neoppod.org/
 .. _ZEO:            https://pypi.python.org/pypi/ZEO
 
-.. relative links are made so they show ok on project main page
-__ wendelin.core/blob/master/demo/demo_zbigarray.py
+__ demo/demo_zbigarray.py