runtime: fix stackDebug comment

Change-Id: Ia9191bd7ecdf7bd5ee7d69ae23aa71760f379aa8
Reviewed-on: https://go-review.googlesource.com/9590
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/runtime/stack1.go b/src/runtime/stack1.go
index 4fa1a58..f74694b 100644
--- a/src/runtime/stack1.go
+++ b/src/runtime/stack1.go
@@ -7,7 +7,7 @@
 import "unsafe"
 
 const (
-	// StackDebug == 0: no logging
+	// stackDebug == 0: no logging
 	//            == 1: logging of per-stack operations
 	//            == 2: logging of per-frame operations
 	//            == 3: logging of per-word updates