twitch: update to new endpoint

Update the OAuth APIs for Twitch: https://discuss.dev.twitch.tv/t/oauth-kraken-migration/14606

Change-Id: I6911d0ba6db7ad07e7ddd277713ec72cc3e00ab5
GitHub-Last-Rev: a3b37aecaf
GitHub-Pull-Request: golang/oauth2#291
Reviewed-on: https://go-review.googlesource.com/113655
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

twitch/twitch.go

@@ -14,6 +14,6 @@ import (
 // For more information see:
 // https://dev.twitch.tv/docs/authentication
 var Endpoint = oauth2.Endpoint{
-	AuthURL:  "https://api.twitch.tv/kraken/oauth2/authorize",
-	TokenURL: "https://api.twitch.tv/kraken/oauth2/token",
+	AuthURL:  "https://id.twitch.tv/oauth2/authorize",
+	TokenURL: "https://id.twitch.tv/oauth2/token",
 }