minor #13995 Add namespace into service_container.rst file (TheGarious)

This PR was submitted for the 5.1 branch but it was merged into the 4.4 branch instead.

Discussion
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Add namespace into service_container.rst file

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Commits
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e553124 Add namespace into service_container.rst file

Author: javiereguiluz

service_container.rst

@@ -331,7 +331,8 @@ Now, this new service is ready to be used. In a controller, for example,
 you can type-hint the new ``SiteUpdateManager`` class and use it::
 
     // src/Controller/SiteController.php
-
+    namespace App\Controller;
+    
     // ...
     use App\Updates\SiteUpdateManager;