os: be more specific on what flags are used for

The flags are used in OpenFile, not Open.

Change-Id: I45c1639e36694529cb29c2b580c43a22e6fd10ac
Reviewed-on: https://go-review.googlesource.com/17352Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -52,8 +52,8 @@ var (
 	Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
 )
 
-// Flags to Open wrapping those of the underlying system. Not all flags
-// may be implemented on a given system.
+// Flags to OpenFile wrapping those of the underlying system. Not all
+// flags may be implemented on a given system.
 const (
 	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
 	O_WRONLY int = syscall.O_WRONLY // open the file write-only.