Please fix the theme tutorial

This tutorial says to implement .Data.Pages inside of a navbar but this variable isn’t available inside of _default/single.html. I managed to fix it by using .Site.Pages instead and it’s best to replace your tutorial with that variable too. Thanks.

This is an open source project, the code and the documentations is built by its users. If you find improvments/errors in the documentaion, please correct it in form of a pull request.