minor #16458 [DependencyInjection] Fix YAML example in "Defining a Service Locator" section (jbieliauskas)

This PR was merged into the 4.4 branch.

Discussion
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[DependencyInjection] Fix YAML example in "Defining a Service Locator" section

Using this example in `v5.3.13` gives this error:

```
Symfony\Component\DependencyInjection\Definition::setArguments(): Argument #1 ($arguments) must be of type array, Symfony\Component\DependencyInjection\Argument\ServiceLocatorArgument given, called in /project/vendor/symfony/dependency-injection/Loader/YamlFileLoader.php on line 527
```

I assume this is because `!service_locator` is the first constructor argument but it's passed as an array of constructor arguments, in which case the example is not right.

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Commits
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bd8b262 Fix YAML example in "Defining a Service Locator" section

Author: javiereguiluz

service_container/service_subscribers_locators.rst

@@ -256,9 +256,10 @@ argument of type ``service_locator``:
         # config/services.yaml
         services:
             App\CommandBus:
-                arguments: !service_locator
-                    App\FooCommand: '@app.command_handler.foo'
-                    App\BarCommand: '@app.command_handler.bar'
+                arguments: 
+                  - !service_locator
+                      App\FooCommand: '@app.command_handler.foo'
+                      App\BarCommand: '@app.command_handler.bar'
 
     .. code-block:: xml