cmd/godoc: use *goroot as base path in zip file

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5781069

src/cmd/godoc/appinit.go

@@ -42,7 +42,7 @@ func init() {
 		log.Fatalf("%s: %s\n", zipfile, err)
 	}
 	// rc is never closed (app running forever)
-	fs.Bind("/", NewZipFS(rc, zipFilename), "/", bindReplace)
+	fs.Bind("/", NewZipFS(rc, zipFilename), *goroot, bindReplace)
 
 	// initialize http handlers
 	readTemplates()