minor twigphp#2562 Change typo in documentation (uthopiko)

This PR was submitted for the master branch but it was merged into the 1.x branch instead (closes twigphp#2562).

Discussion
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Change typo in documentation

In example of use for test_iterable function there is a typo

Commits
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8607f13 Change typo in documentation

lib/Twig/Extension/Core.php

@@ -1444,7 +1444,7 @@ function twig_test_empty($value)
  *
  * <pre>
  * {# evaluates to true if the foo variable is an array or a traversable object #}
- * {% if foo is traversable %}
+ * {% if foo is iterable %}
  *     {# ... #}
  * {% endif %}
  * </pre>