updates README in batch-event sample setting min Java version to 8

Author: dixittarun81

spring-cloud-task-samples/batch-events/README.adoc

@@ -14,7 +14,7 @@ Note: More information on this topic is available https://docs.spring.io/spring-
 
 == Requirements:
 
-* Java 7 or Above
+* Java 8 or Above
 
 == Build: