os: Fix comment in generated signal_unix.go file.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5013042

src/pkg/os/mkunixsignals.sh

@@ -3,7 +3,7 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-echo '// ./mkunix.sh' "$1"
+echo '// ./mkunixsignals.sh' "$1"
 echo '// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT'
 echo