Update datetime.rst

Fixed "the the" typo.

Author: althaus

reference/forms/types/datetime.rst

@@ -74,8 +74,8 @@ format
 
 **type**: ``string`` **default**: ``Symfony\Component\Form\Extension\Core\Type\DateTimeType::HTML5_FORMAT``
 
-If the ``widget`` option is set to ``single_text``, this option specifies the
-the format of the input, i.e. how Symfony will interpret the given input
+If the ``widget`` option is set to ``single_text``, this option specifies
+ the format of the input, i.e. how Symfony will interpret the given input
 as a datetime string. It defaults to the `RFC 3339`_ format which is used
 by the HTML5 ``datetime`` field. Keeping the default value will cause the
 field to be rendered as an ``input`` field with ``type="datetime"``.