fix: use offer url instead of amf app name

[gruyaume]

Description

We fix a bug in the tutorial that was caused by a change in the Terraform modules:

• https://github.com/canonical/terraform-juju-sdcore/pull/59/

Logs

guillaume@thinkpad:~/terraform$ terraform apply --auto-approve
╷
│ Error: Unsupported attribute
│ 
│   on main.tf line 41, in resource "juju_integration" "gnbsim-amf":
│   41:     name     = module.sdcore.amf_app_name
│     ├────────────────
│     │ module.sdcore is a object
│ 
│ This object does not have an attribute named "amf_app_name".
╵
╷
│ Error: Unsupported attribute
│ 
│   on main.tf line 42, in resource "juju_integration" "gnbsim-amf":
│   42:     endpoint = module.sdcore.fiveg_n2_endpoint
│     ├────────────────
│     │ module.sdcore is a object
│ 
│ This object does not have an attribute named "fiveg_n2_endpoint".
╵

Checklist:

• [ ] Documentation follows the Canonical Documentation Style Guide
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have bumped the version of the library

[gruyaume]

Actually this does not work, closing:

│ Error: Client Error
│ 
│   with juju_integration.gnbsim-amf,
│   on main.tf line 32, in resource "juju_integration" "gnbsim-amf":
│   32: resource "juju_integration" "gnbsim-amf" {
│ 
│ Unable to consume remote offer, got error: cannot add saas application "amf": local application with same name already exists
╵