Fix incomplete argo manifest

[wlthomson]

Fixes FD-212.

Step 4 of the docs for "Installing an Argo CD Application" includes a yaml snippet for a manifest spec, sans the outer envelope and parent spec: block.

The wording of the section was originally "The application yaml required to install the helm chart is as follows," but was recently changed to the current phrasing (#3043), which includes an optional instruction to "...commit this manifest to your git-ops repository."

This change updates the yaml block to include the full K8 resource envelope.

[ATGardner]

that's a good point. thanks for spotting this.
i am not entirely sure, but we might also want to add

  syncPolicy:
    automated:
      prune: true
      selfHeal: true
      allowEmpty: false

    syncOptions:
      - PrunePropagationPolicy=foreground
      - Replace=false
      - PruneLast=false
      - Validate=true
      - CreateNamespace=false
      - ApplyOutOfSyncOnly=false
      - RespectIgnoreDifferences=false

to the spec.
it might be a bit too opinionated (assuming the user wants auto-sync with pruning and self-heal). so we can also leave it out, and assume the user knows enough of argo-cd to set up their own preferences on the application.