### 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 <[email protected]>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/415
Reviewed-by: pat-s <[email protected]>
Reviewed-by: justusbunsi <[email protected]>
Co-authored-by: towo <[email protected]>
Co-committed-by: towo <[email protected]>
