minor #18059 Update create_custom_field_type.rst (hbgamra)

This PR was submitted for the 6.3 branch but it was merged into the 6.2 branch instead.

Discussion
----------

Update create_custom_field_type.rst

Adding return type for setNormalizer callback function

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `6.x` for features of unreleased versions).

-->

Commits
-------

d887b5a Update create_custom_field_type.rst

Author: javiereguiluz

form/create_custom_field_type.rst

@@ -271,7 +271,8 @@ to define, validate and process their values::
 
             // optionally you can transform the given values for the options to
             // simplify the further processing of those options
-            $resolver->setNormalizer('allowed_states', static function (Options $options, $states) {
+            $resolver->setNormalizer('allowed_states', static function (Options $options, $states): ?array 
+            {
                 if (null === $states) {
                     return $states;
                 }