Fix link to main.js in webapp example for nested routes

[axeltlarsson]

I came across an issue when using elm-live to serve my Elm app. If I refresh while visiting a nested route such as localhost:8000/elm_rocks/new, the relative uri to include main.js is incorrect as it becomes localhost:8000/elm_rocks/main.js which fails as main.js is served from the root path.

The error I got specifically complains about "X-Content-Type-Options: nosniff".

I'm hoping this small fix can help other Elm beginners use this wonderful language ❤️!