Add missing traverse option in index

Author: l-vo

reference/constraints/Traverse.rst

@@ -10,6 +10,7 @@ constraint.
 | Applies to     | :ref:`class <validation-class-target>`                                              |
 +----------------+-------------------------------------------------------------------------------------+
 | Options        | - `payload`_                                                                        |
+|                | - :ref:`traverse <traverse-option>`                                                 |
 +----------------+-------------------------------------------------------------------------------------+
 | Class          | :class:`Symfony\\Component\\Validator\\Constraints\\Traverse`                       |
 +----------------+-------------------------------------------------------------------------------------+
@@ -142,6 +143,7 @@ Options
 
 The ``groups`` option is not available for this constraint.
 
+.. _traverse-option:
 
 ``traverse``
 ~~~~~~~~~~~~