Fix: Change secret key reference to use 'value' instead of 'key'

[just1900]

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Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] DCO signed
• [ ] Chart Version bumped, please refer to the chart version instruction
• [x] Variables are documented in the values.yaml or README.md
• [x] Title of the PR starts with chart name (e.g. [velero])