minor #12840 Update injection_types.rst (boajer)

This PR was submitted for the 5.0 branch but it was merged into the 4.3 branch instead (closes #12840).

Discussion
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Update injection_types.rst

Typo fixed in the code snippet

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Commits
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43f4422 Update injection_types.rst

Author: xabbuh

service_container/injection_types.rst

@@ -353,7 +353,7 @@ Another possibility is setting public fields of the class directly::
         return function(ContainerConfigurator $configurator) {
             $services = $configurator->services();
 
-            $services->set('app.newsletter_manager, NewsletterManager::class)
+            $services->set('app.newsletter_manager', NewsletterManager::class)
                 ->property('mailer', ref('mailer'));
         };