Fix $nav-desktop in Responsive Navigation Variables

[crystalms]

https://github.com/bu-ist/responsive-foundation/blob/4.0.1/css-dev/burf-theme/layout/_navigation.scss#L740

When customizing the $nav-desktop variable in r-summer, I had to add some styles to override this. I think it should be $nav-desktop instead of $sm (the default setting of $nav-desktop), so if someone enters a different value, this will work as expected.

Also, I didn't see an option for $nav-desktop in the list of Responsive Navigation Variables within r-summer, so I didn't realize it was available until digging through BURF. We might want to add it if it's missing from newer versions?