bug #12100 Fix custom logging formatter (jjanvier)

This PR was merged into the 3.4 branch.

Discussion
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Fix custom logging formatter

When I use the '@' in the service name, I get this kind of error:

```
In CheckExceptionOnInvalidReferenceBehaviorPass.php line 31:

  The service "monolog.handler.deprecation" has a dependency on a non-existent service "@monolog.formatter.deprecation".
```

Commits
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b3828cd Fix custom logging formatter

Author: OskarStark

logging/formatter.rst

@@ -19,7 +19,7 @@ configure your handler to use it:
                 file:
                     type: stream
                     level: debug
-                    formatter: '@monolog.formatter.json'
+                    formatter: 'monolog.formatter.json'
 
     .. code-block:: xml