minor symfony#9000 Handler formatter example was using wrong defined service (gregsomers)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes symfony#9000).

Discussion
----------

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`.

Commits
-------

447844a Handler formatter example was using wrong defined service

Author: javiereguiluz

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
 
@@ -106,7 +106,7 @@ 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"
                 />
             </monolog:config>
         </container>
@@ -132,7 +132,7 @@ 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',
                 ),
             ),
         ));