Update index.adoc for version 5.2

Author: fmbenhassine

spring-batch-docs/modules/ROOT/pages/index.adoc

@@ -9,7 +9,7 @@ xref:spring-batch-intro.adoc[Spring Batch Introduction] :: Background, usage
  scenarios, and general guidelines.
 xref:spring-batch-architecture.adoc[Spring Batch Architecture] :: Spring Batch
 architecture, general batch principles, batch processing strategies.
-xref:whatsnew.adoc[What's new in Spring Batch 5.1] :: New features introduced in version 5.1.
+xref:whatsnew.adoc[What's new in Spring Batch 5.2] :: New features introduced in version 5.2.
 xref:domain.adoc[The Domain Language of Batch] :: Core concepts and abstractions
 of the Batch domain language.
 xref:job.adoc[Configuring and Running a Job] :: Job configuration, execution, and