Update PGP signing plugin details in the summary

Since the recommended plugin is sbt-pgp, it was a little confusing to reach
the bottom of the [Deploying to Sonatype](http://www.scala-sbt.org/release/docs/Community/Using-Sonatype.html#summary) guide and
see references to the `xsbt-gpg-plugin` whereas the [top of the page](http://www.scala-sbt.org/release/docs/Community/Using-Sonatype.html#first-pgp-signatures) talks about using sbt-pgp.
Modify wording to be a bit more clear