Fix modernizr path in docs

The corret path is vendor/modernizr as can be seen in foundation-rails gem:
https://github.com/zurb/foundation-rails/blob/master/vendor/assets/javascripts/vendor/modernizr.js

The path vendor/custom.modernizr gives the following error:
ActionController::RoutingError (No route matches [GET] "/javascripts/vendor/custom.modernizr.js"):
Merge pull request #3679 from leandrocp/master

Fix modernizr path in docs