Get started documentation out of sync with repo

[brennerm]

While going through the get started of Flux I encountered the following issue. In the step Commiting a small change I am told to change the value of the variable PODINFO_UI_MESSAGE which:

a. does not exist in https://github.com/fluxcd/flux-get-started/blob/master/workloads/podinfo-dep.yaml
b. does not seem to be supported by the image stefanprodan/podinfo:3.1.0

What I ended up using was the --ui-message argument for the container.

Feel free to let me know if the mistake is on my side.