fix incorrect phpdoc type hints

src/Annotation/JsonRpcMethod.php

@@ -32,7 +32,7 @@ class JsonRpcMethod extends AnnotationBase implements MethodInterface {
    * Optional. If the method ID is 'foo.bar', this defaults to 'bar'. If the
    * method ID does not contain a dot (.), defaults to 'execute'.
    *
-   * @var string
+   * @var string|null
    */
   public $call;