corrected link to monolog v2.x lib

Author: philkra

README.md

@@ -7,4 +7,4 @@ ECS Logging - Common resources and issues for the language specific ECS loggers
   * [Log4j2](https://github.com/elastic/ecs-logging-java/blob/master/log4j2-ecs-layout/README.md)
   * [Log4j](https://github.com/elastic/ecs-logging-java/blob/master/log4j-ecs-layout/README.md)
 * [PHP](https://github.com/elastic/ecs-logging-php)
-  * [Monolog](https://github.com/elastic/ecs-logging-php/blob/master/docs/Monolog/README.md)
+  * [Monolog v2.x](https://github.com/elastic/ecs-logging-php/blob/master/docs/Monolog_v2.md)