Fix Spring Boot dependency version property name.

The version property used by Spring Boot to manage Spring Data modules pulled in transitively by the Spring Data BOM has changed from 'spring-data-releasetrain.version' to 'spring-data-bom.version'.

Closes #2766

src/main/asciidoc/dependencies.adoc

@@ -48,8 +48,12 @@ You can find a working example of using the BOMs in our https://github.com/sprin
 [[dependencies.spring-boot]]
 == Dependency Management with Spring Boot
 
-Spring Boot selects a recent version of Spring Data modules for you. If you still want to upgrade to a newer version, set
-the `spring-data-releasetrain.version` property to the <<dependencies.train-version,train version and iteration>> you would like to use.
+Spring Boot selects a recent version of the Spring Data modules for you. If you still want to upgrade to a newer version,
+set the `spring-data-bom.version` property to the <<dependencies.train-version,train version and iteration>>
+you would like to use.
+
+See Spring Boot's https://docs.spring.io/spring-boot/docs/current/reference/html/dependency-versions.html#appendix.dependency-versions.properties[documentation]
+(search for "Spring Data Bom") for more details.
 
 [[dependencies.spring-framework]]
 == Spring Framework