minor #10060 Update service_container.rst (loevgaard)

This PR was submitted for the 4.1 branch but it was merged into the 3.4 branch instead (closes #10060).

Discussion
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Update service_container.rst

Wrong definition of alias in service container

Commits
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f1ce22a Update service_container.rst

Author: javiereguiluz

service_container.rst

@@ -1128,7 +1128,7 @@ admin email. In this case, each needs to have a unique service id:
                     <argument>contact@example.com</argument>
                 </service>
 
-                <alias id="AppBundle\Updates\SiteUpdateManager" service="site_update_manager.superadmin" />
+                <service id="AppBundle\Updates\SiteUpdateManager" alias="site_update_manager.superadmin" />
             </services>
         </container>