Fix gcp docs broken link

⚠ ️NOTE THAT THIS FORK IS MAINTAINED FOR CRITICAL BUG FIXES AFFECTING RUNNING COSTS ONLY. NO OTHER CONTRIBUTIONS WILL BE ACCEPTED. ⚠️

What critical bug this MR is fixing?

gitlab-org/gitlab#351269 (comment 867334263)

How does this change help reduce cost of usage? What scale of cost reduction is it?

In what scenarios is this change usable with GitLab Runner's docker+machine executor?