Undefined parameter in pom.xml for StarterPipeline 2.4 Dataflow archetype

[chanseokoh]

GoogleCloudPlatform/google-cloud-eclipse#3043

<maven-exec-plugin.version>@maven-exec-plugin.version@</maven-exec-plugin.version>

Should be exec-maven-plugin (or exec-maven-plugin in other places should be maven-exec-plugin). Let me create a PR.