net: fix comment on Dial with unixgram
net: fix comment on Dial with unixgram

We should use DialUnix or ListenPacket for unixgram networks
because Dial doesn't take a local UnixAddr.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5706043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go