bootstrap-haskell: s/will download/can download/

A one word tweak to weaken the language in the initial explanation to make it "less scary": in general ghcup does not always download all of ghcup, ghc, cabal, stack, and hls (unless requested or they are not already installed, etc), but "will download" sounds like the user has no choice here except to always download everything, which might give them second thoughts about trying this script and hence adopting ghcup.

Perhaps the wording could be made further more precise, but at least "can" gives one less anxiety.