[Form] Replace `an` with `is`

src/Symfony/Component/Form/Extension/Core/DataTransformer/DateTimeToArrayTransformer.php

@@ -143,15 +143,15 @@ class DateTimeToArrayTransformer extends BaseDateTimeTransformer
         }
 
         if (isset($value['month']) && !ctype_digit($value['month'])) {
-            throw new TransformationFailedException('This month an invalid');
+            throw new TransformationFailedException('This month is invalid');
         }
 
         if (isset($value['day']) && !ctype_digit($value['day'])) {
-            throw new TransformationFailedException('This day an invalid');
+            throw new TransformationFailedException('This day is invalid');
         }
 
         if (isset($value['year']) && !ctype_digit($value['year'])) {
-            throw new TransformationFailedException('This year an invalid');
+            throw new TransformationFailedException('This year is invalid');
         }
 
         if (!empty($value['month']) && !empty($value['day']) && !empty($value['year']) && false === checkdate($value['month'], $value['day'], $value['year'])) {