Fixing the redirection in case the slug is in slugs, but is not the head

For some reason, this didn't work in the starter kit.

Example: `/document/UkL0gMuvzYUANCpV/coconut-macaron` should redirect towards `/document/UkL0gMuvzYUANCpV/cool-coconut-macaron`, because for this document, `doc.slugs` equals `["cool-coconut-macaron","coconut-macaron"]`.

I'm sending this as a PR because I have a question, which could lead to an even better way to do it: since we're advocating it as the proper way to write URLs for all cases, shouldn't we make it a method in `PrismicService`, for reusability?