build-vm-configurations missing the info about how to actually select the configuration

[Nic30]

https://github.com/appveyor/website/blob/master/src/docs/build-environment.md#build-vm-configurations

The table of VM build configurations does not have any description.

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As a stupid user with no experience with Hyper-V I was not able to find how to select VM config with more RAM from appveyor.yml after 15 minutes of intense google searching. I think that this information should be exactly missing 2 lines behind the table.