use correct bitgo express quickstart guide URL
use correct bitgo express quickstart guide URL

Summary: The existing link was throwing a 404

Reviewers: zacleids, luis

Reviewed By: luis

Subscribers: ben

Differential Revision: https://phabricator.bitgo.com/D6155

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS