minor #20473 use ? before nullable single type declaration (xabbuh)

This PR was merged into the 7.1 branch.

Discussion
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use ? before nullable single type declaration

see https://github.com/symfony/symfony-docs/actions/runs/12357370718/job/34485453495

Commits
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b0f09c6 use ? before nullable single type declaration

serializer/custom_name_converter.rst

@@ -30,7 +30,7 @@ A custom name converter can handle such cases::
 
     class OrgPrefixNameConverter implements NameConverterInterface
     {
-        public function normalize(string $propertyName, string $class = null, ?string $format = null, array $context = []): string
+        public function normalize(string $propertyName, ?string $class = null, ?string $format = null, array $context = []): string
         {
             // during normalization, add the prefix
             return 'org_'.$propertyName;