go.talks/present: URL use `/` instead of `\` on windows

Fixes golang/go#5876.

R=golang-dev, dave, minux.ma, adg
CC=golang-dev
https://golang.org/cl/11219045
diff --git a/present/dir.go b/present/dir.go
index 12100da..0a04fef 100644
--- a/present/dir.go
+++ b/present/dir.go
@@ -128,7 +128,7 @@
 		}
 		e := dirEntry{
 			Name: fi.Name(),
-			Path: filepath.Join(name, fi.Name()),
+			Path: filepath.ToSlash(filepath.Join(name, fi.Name())),
 		}
 		if fi.IsDir() && showDir(e.Name) {
 			d.Dirs = append(d.Dirs, e)