Fix possible error 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 on `Traversable`.

Author: sp1ker

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'];
         }