Update NPM to point at the repo with the implentation's codebase

[aljones15]

https://www.npmjs.com/package/did-method-key

The HomePage on npm still points at:
http://github.com/digitalbazaar/did-method-key

that should probably be updated to the newer spec repo.

The repo link should probably point here:
https://github.com/digitalbazaar/did-method-key-js

When using the npm page for did-method-key it's not immediately apparent where the code base and/or spec are.

[dlongley]

Closing as the link goes to the right place now.