Update the PHPDoc string

lib/experimental/block-bindings/sources/index.php

@@ -13,8 +13,7 @@
 	 *
 	 * @param string   $source_name The name of the source.
 	 * @param string   $label The label of the source.
-	 * @param callable $apply The callback executed when the source is processed during block rendering. The callable should have the following signature:
-	 *                        function (object $source_attrs, object $block_instance, string $attribute_name): string
+	 * @param callable(object, object, string): string $apply - The callback executed when the source is processed during block rendering.
 	 *                        - object $source_attrs: Object containing source ID used to look up the override value, i.e. {"value": "{ID}"}.
 	 *                        - object $block_instance: The block instance.
 	 *                        - string $attribute_name: The name of an attribute used to retrieve an override value from the block context.