Handler formatter example was using wrong defined service

The service class passed to the formatter in the example handler was using the processor service instead of the formatter service. It should instead use `monolog.formatter.session_request`, otherwise monolog complains that the class does not implement interface `Monolog\Formatter\FormatterInterface`.

Author: gregsomers

logging/processors.rst

@@ -71,7 +71,7 @@ using a processor.
                     type: stream
                     path: '%kernel.logs_dir%/%kernel.environment%.log'
                     level: debug
-                    formatter: app.logger.session_request_processor
+                    formatter: monolog.formatter.session_request
 
     .. code-block:: xml