Add missing data-offcanvas initialization attribute

It appears the documentation markup for the [Off Canvas Multilevel Menu][1] example is missing the data attribute `data-offcanvas`.

The problem has been made apparent from [this StackOverflow question][2]

[1]: http://foundation.zurb.com/docs/components/offcanvas.html#off-canvas-multilevel-menu
[2]: http://stackoverflow.com/questions/26595170/why-doesnt-zurb-foundation-off-canvas-work-in-my-project/26595666#26595666