fix go install command on /installation page

[TopherIsSwell]

the original install command doesn't work with modern versions of golang because it needs to specify version 2:

$ go install github.com/bettercap/bettercap@latest 
go: github.com/bettercap/bettercap@latest: no matching versions for query "latest"

but go install github.com/bettercap/bettercap/v2@latest works