The section states:

The GFE Bundle will be sent as the sole payload of a $gfe-submit operation, which is based on the Asynchronous Interaction Request Pattern (please refer to that page for more details). Note: that page is part of the FHIR R5 current build, but uses no R5 resources; this guide is pre-adopting that HTTP request pattern. The response will be a URL in the Content-Location header for subsequent polling.

The link pointed to in the text  Asynchronous Interaction Request Pattern, points to the FHIR standard CI build http://build.fhir.org/async-bundle.html.

Change this to point to https://hl7.org/fhir/R5/async-bundle.html