### Description of the change
Fix a wrong reference to `signing.existingKey`, `signing.existingSecret` was what was meant and what is used in the chart.
### Benefits
Less confusion when trying to use the Helm chart.
### Possible drawbacks
Evangelists of `existingKey` storming the barricades even though `existingKey` is long dead.
### Applicable issues
None, nobody noticed enough to care, apparently.
Co-authored-by: Tobias Wolter <towo@towo.eu>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/415
Reviewed-by: pat-s <pat-s@noreply.gitea.io>
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-authored-by: towo <towo@noreply.gitea.io>
Co-committed-by: towo <towo@noreply.gitea.io>
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