### Description of the change
Hello, following the documentation, I saw that there was a small error concerning the declaration of a PVC on the PostgreSQL part.
So I just changed this value from claimName to existingClaim
### Applicable issues
- Fixes#778
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/786
Reviewed-by: justusbunsi <[email protected]>
Co-authored-by: yael-tramier <[email protected]>
Co-committed-by: yael-tramier <[email protected]>
