Fixed the example so it works out of the box

Without the line 29 changes (line 39 in the README):

$ go run backup_dyn_zones.go 
# command-line-arguments
./backup_dyn_zones.go:29: zone.Zone undefined (type string has no field or method Zone)
./backup_dyn_zones.go:29: zone.Serial undefined (type string has no field or method Serial)