[docs/examples] update examples/helm-deployment (and other examples if needed) to reflect setValueTemplates preference (vs setValues)

Issues tracks updates examples/helm-deployment (and other examples if needed) to reflect setValueTemplates preference (vs setValues). Currentlyt the example has the following `skaffold.yaml`:
```
apiVersion: skaffold/v4beta1
kind: Config
build:
artifacts:
- image: skaffold-helm
deploy:
helm:
releases:
- name: skaffold-helm
chartPath: charts
setValues:
image: skaffold-helm
```

This is confusing as it seems to suggest `setValues` usage for image subst. which is not correct to do (causes errors w/ multiple images). It should instead be:
```
apiVersion: skaffold/v4beta1
kind: Config
build:
artifacts:
- image: skaffold-helm
deploy:
helm:
releases:
- name: skaffold-helm
chartPath: charts
setValueTemplates:
image: {{.IMAGE_FULLY_QUALIFIED_skaffold_helm}}
```