Linkage Checker Gradle plugin instruction to depend on the build phase

[suztomo]

As of now, the plugin documentation doesn't add any task dependency.

https://github.com/GoogleCloudPlatform/cloud-opensource-java/wiki/Linkage-Checker-with-Gradle

However linkageCheck task expects the project's JAR file built before running it. Therefore it should depend on "build" task.

[suztomo]

For now, I updated the documentation

// Run build task first to generate JAR file of this project
linkageCheck.dependsOn('build')