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

Karthikvb/talks,Karthikvb/talks,Karthikvb/talks,Karthikvb/talks,Karthikvb/talks,Karthikvb/talks