[IntegerType] fix rounding_mode default value

change default value of rounding_mode option from \NumberFormatter::ROUND_HALFUP to default: \NumberFormatter::ROUND_DOWN 
see https://github.com/symfony/symfony/blob/6.0/src/Symfony/Component/Form/Extension/Core/Type/IntegerType.php::49

Author: AboulhajHakim

reference/forms/types/integer.rst

@@ -35,7 +35,7 @@ Field Options
 ``rounding_mode``
 ~~~~~~~~~~~~~~~~~
 
-**type**: ``integer`` **default**: ``\NumberFormatter::ROUND_HALFUP``
+**type**: ``integer`` **default**: ``\NumberFormatter::ROUND_DOWN``
 
 By default, if the user enters a non-integer number, it will be rounded
 down. You have several configurable options for that rounding. Each option