minor #7222 Replace undefined argument with intended string argument (gskema)

This PR was merged into the 2.7 branch.

Discussion
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Replace undefined argument with intended string argument

Since the comment mentions `app.user_config_manager`, I assume it was meant to be used as an argument here.

Commits
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7345297 Replace undefined argument with intended string argument

Author: xabbuh

service_container/definitions.rst

@@ -29,7 +29,7 @@ There are some helpful methods for working with the service definitions::
     // get the "app.user_config_manager" definition
     $definition = $container->getDefinition('app.user_config_manager');
     // get the definition with the "app.user_config_manager" ID or alias
-    $definition = $container->findDefinition($serviceId);
+    $definition = $container->findDefinition('app.user_config_manager');
 
     // add a new "app.number_generator" definitions
     $definition = new Definition('AppBundle\NumberGenerator');