unix: fix example
unix: fix example

This should call unix.Exec, not syscall.Exec. Thanks Tobias Klauser
for the spot.

Change-Id: Iddae390891a66652b071e05c1a5a25bbfdb19e52
Reviewed-on: https://go-review.googlesource.com/101435
Run-TryBot: Kevin Burke <685caccd7011cfe58b8f9ef79ff9dca388f03c79@inburke.com>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys