Minor cleanup and index.js removal.

1. Fixed a typo in the description field.
2. Removed useless `engines` field (it just says that all Node.js versions are OK). There was also a `engine` field (note missing `s`). I've fixed this.
3. Useless `index.js` file. I've removed this and pointed `main` in `package.json` to `ipv6.js`.