ipv6: fix a typo

Change-Id: I4ce6dfec994252fec3874fb748e0d6b8db97394d
Reviewed-on: https://go-review.googlesource.com/21835
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ipv6/gen.go

@@ -52,7 +52,7 @@ func genzsys() error {
 	if err != nil {
 		return err
 	}
-	// The ipv6 pacakge still supports go1.2, and so we need to
+	// The ipv6 package still supports go1.2, and so we need to
 	// take care of additional platforms in go1.3 and above for
 	// working with go1.2.
 	switch {