Please update installation instructions

Should be:

```text
hugo new site mynewsite
cd mynewsite/
git init
git submodule add https://github.com/zerostaticthemes/hugo-serif-theme.git themes/hugo-serif-theme
rm -f hugo.toml 
cp -a themes/hugo-serif-theme/exampleSite/. .
hugo server
```

Or, you could rename the config file in the example site from `config.toml` to `hugo.toml`.



