Update instructions to migrate non-platform machines

[catflydotio]

For an existing Machines app with no unique Machines (all running the same image with the same config)

$ fly config save -a appname # to get a fly.toml from a machine's config
$ fly machine update --metadata fly_platform_version=v2 9080eee5a1d518 # do this for each machine 
$ fly deploy

For an app with Machines running unique images, you can leave some Machines off the platform (so fly deploy doesn't stomp them) but the fly config save command may save the wrong config into fly.toml so you should hand-create a fly.toml instead.

https://github.com/superfly/docs/blob/main/apps/migrate-to-v2.html.md

[catflydotio]

Fixed in 3c5c1d4