windows download: fix wordings

[haraldschilly]

windows-download says

Below are the VirtualBox images

but there are actually the installers.

should we also add a page for the (previous) OVA images? or are they deprecated and it's better to use the windows installer?

[slel]

Yes, the wording should be modified.

Regarding the ova images, I don't think they are, or should be,
deprecated. They can be useful for a number of platforms
not supported by SageMath in any other way.

@vbraun, is there a way you could resume producing these
ova images? Users occasionally ask for an updated version
on the support channels.

[kcrisman]

Agreed that the ova should not be completely deprecated, but also that we are not producing them currently.