runtime: Fix typo in dup3 fallback implementation.
runtime: Fix typo in dup3 fallback implementation.

From Uros Bizjak.

R=iant
CC=gofrontend-dev
https://golang.org/cl/14850046

anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,anlhord/gofrontend,anlhord/gofrontend