Clarify example.

The previous example worked but relied on a behavior of xmlrpclib to
use an implied path of "/RPC2" when no path is given (note that it had
no trailing slash).  The new example explicitly uses "/RPC2".

docs/introduction.rst

@@ -165,7 +165,7 @@ XML-RPC Interface
   .. code-block:: python
 
      import xmlrpclib
-     server = xmlrpclib.Server('http://localhost:9001')
+     server = xmlrpclib.Server('http://localhost:9001/RPC2')
           
   You may call methods against the :program:`supervisord` and its
   subprocesses by using the ``supervisor`` namespace.  An example is