Update phpstan-param annotation in Feature Manager

The phpstan-param annotation in the add_features method of the wp-404-caching plugin's Feature Manager was updated. This modification improves code clarity by specifying 'string' as the key type of the first-level array in the parameter, ensuring clearer type hinting for developers and better code comprehension.

src/class-feature-manager.php

@@ -25,7 +25,7 @@ class Feature_Manager {
 	 *
 	 * @param array $features_to_create Array of feature classnames and arguments.
 	 *
-	 * @phpstan-param array{string: array{string?: mixed}}|array{} $features_to_create
+	 * @phpstan-param array<string, array{string?: mixed}>|array{} $features_to_create
 	 */
 	public static function add_features( array $features_to_create = [] ): void {
 		foreach ( $features_to_create as $feature_class => $args ) {