Rename exercise `find-needle` to `find-word`

Rename [Exercise 04](https://github.com/CodeYourFuture/js-exercises-tdd/tree/master/II.pass-tests/04-find-needle)

This is to avoid confusion with the contents of the input array, which contains an element named `needle`.

Start with the Exercise 04 directory linked in the issue and inspect its contents and repository references to `find-needle`. Rename the exercise to `find-word`, update references that would otherwise break, and confirm the exercise and its links use the new name.

Written by the indexing model from the issue text.