encoding/gob: fix "// +build" comment for debug.go

R=golang-dev, rsc, adg, r, r
CC=golang-dev
https://golang.org/cl/5693060
diff --git a/src/pkg/encoding/gob/dump.go b/src/pkg/encoding/gob/dump.go
index e23a11e..17238c9 100644
--- a/src/pkg/encoding/gob/dump.go
+++ b/src/pkg/encoding/gob/dump.go
@@ -7,6 +7,7 @@
 package main
 
 // Need to compile package gob with debug.go to build this program.
+// See comments in debug.go for how to do this.
 
 import (
 	"encoding/gob"