Update $_required typehint (#1568)

src/Annotations/AbstractAnnotation.php

@@ -49,7 +49,7 @@ abstract class AbstractAnnotation implements \JsonSerializable
     /**
      * The properties which are required by [the spec](https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md).
      *
-     * @var array
+     * @var string[]
      */
     public static $_required = [];