Update CronExpression.php (#158)

Added "throws" docblock section, so it's immediately obvious in which way an invalid cron expression will fail

Co-authored-by: Roy Garrido <[email protected]>

src/Cron/CronExpression.php

@@ -177,6 +177,7 @@ public static function isValidExpression(string $expression): bool
      *
      * @param string $expression CRON expression (e.g. '8 * * * *')
      * @param null|FieldFactoryInterface $fieldFactory Factory to create cron fields
+     * @throws InvalidArgumentException
      */
     public function __construct(string $expression, FieldFactoryInterface $fieldFactory = null)
     {