add explicit @return annotation for mixed type to fix PHP7.4 deprecation notice

Author: Angatar

src/Schema/Rdf/RdfResource.php

@@ -404,6 +404,8 @@ public function offsetExists($offset): bool
 
     /**
      * Array Access Interface: perform get at using array syntax.
+     *
+     * @return mixed Can return all value types.
      */
     #[\ReturnTypeWillChange]
     public function offsetGet($offset)