Fix userguide for projectedVolumes

[Akila-I]

Description

Deploying an OpenSearch cluster with projected volumes as described in the current documentation leads to the following error

OpenSearchCluster.opensearch.opster.io "opensearch" is invalid: spec.general.additionalVolumes[1].projected.sources: Invalid value: "object": spec.general.additionalVolumes[1].projected.sources in body must be of type array: "object"

This PR corrects the userguide on how to add a projected volume to OpenSearchCluster CRD.

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

Check List

• Commits are signed per the DCO using --signoff
• Unittest added for the new/changed functionality and all unit tests are successful
• Customer-visible features documented
• No linter warnings (make lint)

If CRDs are changed:

• CRD YAMLs updated (make manifests) and also copied into the helm chart
• Changes to CRDs documented

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[swoehrl-mw]

Hi @Akila-I. Thanks for noticing this error and opening the PR.
LGTM