[scripts] gitian-builder.sh: fix --setup doc, LXC is default

contrib/gitian-build.sh

@@ -47,7 +47,7 @@ Options:
 -j		Number of processes to use. Default 2
 -m		Memory to allocate in MiB. Default 2000
 --kvm           Use KVM instead of LXC
---setup         Set up the Gitian building environment. Uses KVM. If you want to use lxc, use the --lxc option. Only works on Debian-based systems (Ubuntu, Debian)
+--setup         Set up the Gitian building environment. Uses LXC. If you want to use KVM, use the --kvm option. Only works on Debian-based systems (Ubuntu, Debian)
 --detach-sign   Create the assert file for detached signing. Will not commit anything.
 --no-commit     Do not commit anything to git
 -h|--help	Print this help message