Update virtual-machine.md

[BennieCopeland]

Update VM docs to add missing add_availability_zone option.

I have read the contributing guidelines and have completed the following:

• [ ] Tested my code end-to-end against a live Azure subscription.
• [X] Updated the documentation in the docs folder for the affected changes.
• [ ] Written unit tests against the modified code that I have made.
• [ ] Updated the release notes with a new entry for this PR.
• [ ] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

Docs update