Fix the README about the custom style.css

Using the provided example to include a custom css file
results in this error when trying to serve the site:

```
Error: Error building site:
failed to render pages:
render of "home" failed:
execute of template failed:
template: index.html:5:5:
executing "index.html" at <partial "head" .>:
error calling partial: "/path/to/site/themes/hugo-split-theme/layouts/partials/head.html:27:11":
execute of template failed:
template: partials/head.html:27:11:
executing "partials/head.html" at <.Site.Params.custom.css.enable>:
can't evaluate field enable in type []map[string]interface {}
```

Using [params.custom.css] instead of [[params.custom.css]]
solves this issue. The custom file is included and no error
occurs when serving the site.