updated mvn build command for native build because it was outdated

Author: ntschutta

spring-cloud-task-samples/timestamp/README.adoc

@@ -29,7 +29,7 @@ java -jar target/timestamp-task-3.0.0.jar
 
 [source,shell]
 ----
-mvn -Pnative clean package
+mvn -Pnative native:compile
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 == Native Run: