Fix path operator in Windows for index.html with multi lang

In Windows:

currently `path.join` used in `template.js` results in index.html for multi lang gitbooks generated such as:

    <li>
        <a href="./en\index.html">English</a>
    </li>

    <li>
        <a href="./de\index.html">Deutsch</a>
    </li>

    <li>
        <a href="./fr\index.html">Français</a>
    </li>



This pull request uses `links.join` instead of `path.join` to resolve this issue.