dns/dnsmessage: fix a typo

Change-Id: I288f01233defe6c673ef9dd5ec79353d3663a9af
Reviewed-on: https://go-review.googlesource.com/46790
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed