Fix 'Home' and logo links

Even though the root file for a website is usually called `index`, you
shouldn't actually link to `/index` when you try to go there. The index
should be hosted at `/`, not `/index`.

For reference, see [at what path we're rendering the index file?](https://github.com/berkompasacademy/omnicorp_project/blob/78117dbc468f6c7547ef7a0dae4a1f5bee4c1cf5/app.rb#L7-L9)