Fix renovate for registry version

[sxd]

After #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": []
}