Fix persistance values at PostgreSQL charts.

[rdziwinski]

According with: https://github.com/bitnami/charts/blob/master/bitnami/postgresql/values.yaml

Description of the change

Benefits

Possible drawbacks

Applicable issues

• fixes #

Additional information

Checklist

• [ ] Title of the PR starts with chart name (e.g. [home-assistant])
• [ ] Chart version bumped in Chart.yaml according to semver.
• [ ] Chart artifacthub.io/changes changelog annotation has been updated in Chart.yaml. See Artifact Hub documentation for more info.
• [ ] Variables have been documented in the values.yaml file.