Update recommended VM in docs

[helendduncan]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates
• [x] This is a request for a change to documentation
• [x] This isn't a feature request or bug report
• [x] This isn't an open-ended question (open a discussion if it is).

:ledger: Where is the problem?

The Standard_D8s_v6 VM is not available on the current SHM subscription. The docs suggest to use it VM

:memo: Required documentation changes

Update to VM that is available for the current subscription - (Standard_D8s_v4 and Standard_D8s_v5 are available)

[jemrobinson]

While this is true for the subscription you're using, I'm not sure this SKU is unavailable in general.

[JimMadge]

The docs say Dsv6 is in preview, which I assume means not generally available.