minor #14729 [DependencyInjection] Fix possible error in example code (sp1ker)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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[DependencyInjection] Fix possible error in example code

Pseudo-type `iterable` accepts any `array` or `object` implementing the `Traversable` interface.  Constructor will be crashed when we try to pass array instead of `Traversable` and use `iterator_to_array`.
`iterator_to_array` dont work with arrays.
Then if always passing `Traversable` we could write like this:
```php
public function __construct(Traversable $handlers)
```
or check instance of `Traversable`.

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Commits
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74d460e Fix possible error code

Author: javiereguiluz

service_container/tags.rst

@@ -798,7 +798,7 @@ array element. For example, to retrieve the ``handler_two`` handler::
     {
         public function __construct(iterable $handlers)
         {
-            $handlers = iterator_to_array($handlers);
+            $handlers = $handlers instanceof \Traversable ? iterator_to_array($handlers) : $handlers;
 
             $handlerTwo = $handlers['handler_two'];
         }