x/playground: Cmd.Output example return error from playground

[tomheng]

the Cmd.Output example return error now.

2009/11/10 23:00:00 exec: "date": executable file not found in $PATH

I did not knew if it run successfully before the playgound upgrade to go1.5.
but, it should run successfully as expected.

[adg]

It won't run in the playground because the playground can't execute processes.