minor #3473 Fix length filter documentation about computation (VincentLanglet)

This PR was merged into the 1.x branch.

Discussion
----------

Fix length filter documentation about computation

According to https://github.com/twigphp/Twig/blob/1.x/src/Extension/CoreExtension.php#L1317

Commits
-------

473f73e Fix length filter doc

Author: fabpot

doc/filters/length.rst

@@ -14,7 +14,7 @@ return value of the ``count()`` method.
 For objects that implement the ``__toString()`` magic method (and not ``Countable``),
 it will return the length of the string provided by that method.
 
-For objects that implement the ``IteratorAggregate`` interface, ``length`` will use the return value of the ``iterator_count()`` method.
+For objects that implement the ``Traversable`` interface, ``length`` will use the return value of the ``iterator_count()`` method.
 
 .. code-block:: twig