overrides/stackspin-nextcloud-override: fix external storage app slug

https://docs.nextcloud.com/server/latest/admin_manual/occ_command.html#files-external-label

overrides/stackspin-nextcloud-override.yaml

@@ -69,7 +69,7 @@ data:
           enabled: false
         - name: contacts
           enabled: true
-        - name: external_storage
+        - name: files_external
           enabled: true
         # Common Apps
         #- name: external