Replace claimName value to existingClaim to manage your PVC value for PostgreSQL chart. (#786)

### 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 <justusbunsi@noreply.gitea.com>
Co-authored-by: yael-tramier <yael-tramier@noreply.gitea.com>
Co-committed-by: yael-tramier <yael-tramier@noreply.gitea.com>

README.md

@@ -576,7 +576,7 @@ You can interact with the postgres settings as displayed in the following exampl
 postgresql:
   persistence:
     enabled: true
-    claimName: MyAwesomeGiteaPostgresClaim
+    existingClaim: MyAwesomeGiteaPostgresClaim
 ```
 
 ### Admin User