os/exec: docs bugs sections points to closed issue

[andrewchambers]

https://golang.org/pkg/os/exec/#pkg-note-BUG points to a closed issue. Should this be addressed?

[minux]

I think that's ok. The bug still exists on 10.6, technically. The issue is closed because we are not going to fix it as we don't support 10.6 anymore. I added the unfortunate label to #2603 to signal that it has not been fixed but we don't care anymore.

[rakyll]

Bug is listed as a note, not for the purpose of getting it addressed.