ChangeOwnerLimitationMapper: Fix phpDoc

Fix  ChangeOwnerLimitationMapper::getSelectionChoices phpDoc

Unable to parse file "vendor/ibexa/admin-ui/src/lib/Limitation/Mapper/ChangeOwnerLimitationMapper.php", an error was detected: An array can have only integers or strings as keys

When used as key, `null` is casted to an empty string.
 `ChangeOwnerLimitation::LIMITATION_VALUE_SELF` should never be `null`.

src/lib/Limitation/Mapper/ChangeOwnerLimitationMapper.php

@@ -64,7 +64,7 @@ public function filterLimitationValues(Limitation $limitation): array
     }
 
     /**
-     * @return array<?int, string>
+     * @return array<int, string>
      */
     private function getSelectionChoices(): array
     {