Fix typo in advanced configuration docs

fix an typo

Two more issues found in the vendor folder (leaving them alone):

/Users/admin/Code/gitlab-runner/vendor/cloud.google.com/go/storage/storage.go:
  974: 	// MediaLink is an URL to the object's content. This field is read-only.

/Users/admin/Code/gitlab-runner/vendor/gitlab.com/gitlab-org/gitlab-terminal/helpers.go:
   57: // in an URL with `[FILTERED]`