Change example to bittorrent-dht-sodium

[emilbayes]

What is the purpose of this pull request? (put an "X" next to item)

[x] Documentation update [ ] Bug fix [ ] New feature [ ] Other, please explain:

What changes did you make? (Give an overview)

This module supports prebuilds and latest node versions

Which issue (if any) does this pull request address?

The documentation was wrong in that sodium signatures do work, however the private key encoding is slightly different. This change is purely documentation

Is there anything you'd like reviewers to focus on?

[mafintosh]

/cc @feross unsure if you have opionions about this

[feross]

@mafintosh Looks good to me