cmd/godoc: Fix reg test on Windows.
cmd/godoc: Fix reg test on Windows.

R=bradfitz, gri
CC=golang-dev
https://codereview.appspot.com/11551043

t-yuki/go.tools.old,tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools