Fix @REST\Collection(hydrator=) param description.

Collection hydrator defaults to `ZfrRest\Stdlib\Hydrator\PaginatorHydrator`, not a (single item) resource hydrator.

Source: [CollectionResourceMetadata.php#L44](https://github.com/Thinkscape/zfr-rest/blob/master/src/ZfrRest/Resource/Metadata/CollectionResourceMetadata.php#L44)