Documentation fixes

There are three doc fixes for builders.py  in this change.

Link to Gradle plugin fixed
Maven-target example has been fixed
Copyartifact docs cleaned up

Change-Id: Ie50e94ace2041735cc986eb117597262b40b0fa5
Reviewed-on: https://review.openstack.org/25962
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins