go/doc: hide "Program exited." message from front page playground output
go/doc: hide "Program exited." message from front page playground output

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/6934049

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang