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>

dns/dnsmessage/message.go

@@ -1209,7 +1209,7 @@ func (b *Builder) Finish() ([]byte, error) {
 	return b.msg, nil
 }
 
-// An ResourceHeader is the header of a DNS resource record. There are
+// A ResourceHeader is the header of a DNS resource record. There are
 // many types of DNS resource records, but they all share the same header.
 type ResourceHeader struct {
 	// Name is the domain name for which this resource record pertains.