godoc: add forgotten file from last CL to fix build

R=adg, franciscossouza, dave
CC=golang-dev
https://golang.org/cl/25650043

cmd/godoc/main.go

@@ -263,7 +263,7 @@ func main() {
 			log.Fatal(err)
 		}
 		index, _ := corpus.CurrentIndex()
-		err = index.Write(f)
+		_, err = index.WriteTo(f)
 		if err != nil {
 			log.Fatal(err)
 		}