Update quickstart-java.md , corrected Kotlin syntax (#32676)

previous syntax for the snippet was in Groovy, but in step 1&2 of the Gradle conversion we use Kotlin

Author: razvanculea

website/www/site/content/en/get-started/quickstart-java.md

@@ -139,9 +139,9 @@ if (project.hasProperty("dataflow-runner")) {
       {{< /highlight >}}
    4. At the end of the build script, add the following task:
       {{< highlight >}}
-task execute (type: JavaExec) {
-    classpath = sourceSets["main"].runtimeClasspath
-    mainClass.set(System.getProperty("mainClass"))
+tasks.register<JavaExec>("execute") {
+  mainClass.set(System.getProperty("mainClass"))
+  classpath = sourceSets.main.get().runtimeClasspath
 }
       {{< /highlight >}}
 4. Build your project: