Update broken URL in Exec Plugin Tutorial
Update broken URL in Exec Plugin Tutorial

- link created in the scripts of Exec plugin on linux in 60 seconds to download the kustomize executable does not work
- used the [releases page](https://github.com/kubernetes-sigs/kustomize/releases/tag/v3.0.0) to fix

kubernetes-sigs/kustomize,kubernetes-sigs/kustomize,kubernetes-sigs/kustomize