Missing import in example

`import logging` is required in the example.

Change-Id: I92777da13b64592d4fb064c3ee0fc9507360ca0f

docs/source/logging.rst

@@ -72,6 +72,7 @@ code.  Using the logging framework is very simple.
 
     # myapp/myapp/controllers/root.py
     from pecan import expose
+    import logging
 
     logger = logging.getLogger(__name__)