[Logger] Adding app channel

Page: https://symfony.com/doc/6.4/logging/channels_handlers.html

Author: ThomasLandauer

logging/channels_handlers.rst

@@ -2,7 +2,7 @@ How to Log Messages to different Files
 ======================================
 
 The Symfony Framework organizes log messages into channels. By default, there
-are several channels, including ``doctrine``, ``event``, ``security``, ``request``
+are several channels, including ``app``, ``doctrine``, ``event``, ``security``, ``request``
 and more. The channel is printed in the log message and can also be used
 to direct different channels to different places/files.