update front-end link in README
update front-end link in README

I just moved the repo to GitHub, so I'm just updating the link :+1:
google/xi-editor,google/xi-editor,modelorganism/xi-editor,google/xi-editor,modelorganism/xi-editor,google/xi-editor,modelorganism/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor