route: update package docs

It's confirmed that this package works on FreeBSD 12-STABLE kernels.

Change-Id: I1f304d3885a351c8c9332b18d6842d1d8d6a66da
Reviewed-on: https://go-review.googlesource.com/c/157277
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

route/route.go

@@ -8,8 +8,8 @@
 // packet routing facilities on BSD variants.
 //
 // The package supports any version of Darwin, any version of
-// DragonFly BSD, FreeBSD 7 through 11, NetBSD 6 and above, and
-// OpenBSD 5.6 and above.
+// DragonFly BSD, FreeBSD 7 and above, NetBSD 6 and above, and OpenBSD
+// 5.6 and above.
 package route
 
 import (