Fix pw(8) syntax for group changes

Changing group gid's doesn't work currently on FreeBSD 8.3 and 9.0.

According to the [pw(8) manual](http://www.freebsd.org/cgi/man.cgi?query=pw&apropos=0&sektion=8&manpath=FreeBSD+9.1-RELEASE&arch=default&format=html) the correct syntax is `pw NAME -g NEWGID`, and salt currently tries to run `pw -g NEWGID NAME`, which fails.

After changing the invocation of `pw` all gid changes work correctly.