Update comments in the prelude

The comments say that the prelude imports std::io::println since it would
be annoying to have to import it in every program that uses it. However,
the prelude doesn't actually import that function anymore. So, update the
comments to better match reality.
auto merge of #12410 : DaGenix/rust/fix-incorrect-comment, r=alexcrichton

The comments say that the prelude imports std::io::println since it would
be annoying to have to import it in every program that uses it. However,
the prelude doesn't actually import that function anymore. So, update the
comments to better match reality.