JCore-Brownfield-AppAttachVM-Fix PS and Azure CLI in readme

[JCoreMS]

Add var to PS and additional Azure CLI code block.

Overview/Summary

Fix PowerShell deployment and Azure CLI deployment code as it was non-functional. Per Issue #674

This PR fixes/adds/changes/removes

1. PowerShell code in readme for brownfield deployment for App Attach VM revised.
2. Azure CLI code in readme for brownfield deployment for App Attach VM revised.

Breaking Changes

1. N/A

Testing Evidence

Deployment tested successfully using either mechanisms.

As part of this Pull Request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Ensured the resource API versions in .bicep file/s I am adding/editing are using the latest API version possible
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues
• (AVD LZA Team Only) Associated it with relevant ADO Items
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Updated relevant and associated documentation (e.g. Contribution Guide, Module READMEs, Docs etc.)