minor #17363 [Notifier] Fix notifier.flash_message_importance_mapper service definition (nlemoine)

This PR was merged into the 6.1 branch.

Discussion
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[Notifier] Fix `notifier.flash_message_importance_mapper` service definition

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Commits
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5dbace5 Fix `notifier.flash_message_importance_mapper` service definition

Author: javiereguiluz

notifier.rst

@@ -717,7 +717,8 @@ typical alert levels, which you can implement immediately using:
 
         # config/services.yaml
         services:
-            notifier.flash_message_importance_mapper: Symfony\Component\Notifier\FlashMessage\BootstrapFlashMessageImportanceMapper
+            notifier.flash_message_importance_mapper: 
+                class: Symfony\Component\Notifier\FlashMessage\BootstrapFlashMessageImportanceMapper
 
     .. code-block:: xml