[Serializer] Add return hint

Return hint in NameConverterInterface added in V6.0

Author: mohamedGasmii

components/serializer.rst

@@ -588,12 +588,12 @@ A custom name converter can handle such cases::
 
     class OrgPrefixNameConverter implements NameConverterInterface
     {
-        public function normalize(string $propertyName)
+        public function normalize(string $propertyName): string
         {
             return 'org_'.$propertyName;
         }
 
-        public function denormalize(string $propertyName)
+        public function denormalize(string $propertyName): string
         {
             // removes 'org_' prefix
             return 'org_' === substr($propertyName, 0, 4) ? substr($propertyName, 4) : $propertyName;