go.talks/pkg/present: remove tilde from bad url chars
go.talks/pkg/present: remove tilde from bad url chars

Obviously we should be able to use tilde in URLs.

R=golang-dev, minux.ma, r, noah.evans
CC=golang-dev
https://codereview.appspot.com/7443052

webfd/go-zh.talks,d0f/go-zh.talks,webfd/go-zh.talks,d0f/go-zh.talks,webfd/go-zh.talks,webfd/go-zh.talks,webfd/go-zh.talks,bryanxu/go-zh.talks,webfd/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks,bryanxu/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks,d0f/go-zh.talks,bryanxu/go-zh.talks,bryanxu/go-zh.talks,d0f/go-zh.talks