Fix renovate for registry version

After https://github.com/cloudnative-pg/plugin-barman-cloud/pull/778 was merged, the line created for the renovate configuration didn't work and failed with:
```
{
"currentValue": "2",
"datasource": "docker",
"depName": "registry",
"packageName": "registry",
"replaceString": "# renovate: datasource=docker depName=registry versioning=semver\n REGISTRY_VERSION: 2\n",
"skipReason": "invalid-value",
"versioning": "semver",
"warnings": [],
"updates": []
}
```