ninja: fix yaml syntax

basic/apps/ninja/invoiceninja-values-configmap.yaml

@@ -14,6 +14,8 @@ data:
       tls: true
       certManager: true
       annotations:
+        nginx.ingress.kubernetes.io/configuration-snippet: |
+          add_header "Content-Security-Policy: frame-ancestors 'self' ${nextcloud_domain}";
     persistence:
       public:
         existingClaim: invoiceninja-data