docs: Update usage example

[mingnz]

Fixed usage example based on Terraform registry entry https://registry.terraform.io/modules/terraform-ibm-modules/iam-access-group/ibm/latest

Description

Fixes the usage example in the README.md, as the current one does not work.

Release required?

• No release
• Patch release (x.x.X)
• Minor release (x.X.x)
• Major release (X.x.x)

Release notes content

Fixes usage example in README.

Run the pipeline

If the CI pipeline doesn't run when you create the PR, the PR requires a user with GitHub collaborators access to run the pipeline.

Run the CI pipeline when the PR is ready for review and you expect tests to pass. Add a comment to the PR with the following text:

/run pipeline

Checklist for reviewers

• If relevant, a test for the change is included or updated with this PR.
• If relevant, documentation for the change is included or updated with this PR.

For mergers

• Use a conventional commit message to set the release level. Follow the guidelines.
• Include information that users need to know about the PR in the commit message. The commit message becomes part of the GitHub release notes.
• Use the Squash and merge option.

[jor2]

Is latest a valid value here for version? Replace with a specific version if not