Update Using-Sonatype.rst

according to this

http://www.scala-sbt.org/sbt-pgp/usage.html#publishing_artifacts

"The PGP plugin NO LONGER wires into the default publish and publish-local tasks of sbt. If you want to published signed artifacts, you must use the new publish-signed and publish-local-signed tasks."

This bit me when i tried to do this today, should update the docs to refer to the proper command `publish-signed` so the next guy who tries this doesn't get bitten.