Fixed Logstash event format link (#898)

src/Monolog/Formatter/LogstashFormatter.php

@@ -15,7 +15,7 @@ namespace Monolog\Formatter;
  * Serializes a log message to Logstash Event Format
  *
  * @see http://logstash.net/
- * @see https://github.com/logstash/logstash/blob/master/lib/logstash/event.rb
+ * @see https://github.com/elastic/logstash/blob/master/logstash-core-event/lib/logstash/event.rb
  *
  * @author Tim Mower <timothy.mower@gmail.com>
  */