minor #15365 [DependencyInjection] Update service_container.rst (noniagriconomie)

This PR was merged into the 4.4 branch.

Discussion
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[DependencyInjection] Update service_container.rst

Fix service declaration

Commits
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e638cf3 Update service_container.rst

Author: javiereguiluz

service_container.rst

@@ -645,7 +645,7 @@ But, you can control this and pass in a different logger:
             // ... same code as before
 
             // explicitly configure the service
-            $services->set(SiteUpdateManager::class)
+            $services->set(MessageGenerator::class)
                 ->arg('$logger', ref('monolog.logger.request'))
             ;
         };