Fix 'Deploying to Sonatype' links

The link to the Sonatype documentation is outdated and the tailing URL of hkp://pool.sks-keyservers.net/ will lead to an error in sbt-pgp:

```
INF: [console logger] dispatch: pool.sks-keyservers.net POST /:11371/pks/add HTTP/1.1
Failed to run pgp-cmd: SendKey(<id>,hkp://pool.sks-keyservers.net/).  
Please report this issue at http://github.com/sbt/sbt-pgp/issues
```