Tiny tweak: use <kbd> instead of <span class="keystroke">

[chriskrycho]

This is a small change which will give improved accessibility for the handful of places we use <span class="keystroke">, e.g. <span class="keystroke">ctrl-c</span>. Right now, there is no CSS or anything associated with that, and since it is just a span with no other information, screen readers won’t make anything of it either. Using <kbd> will automatically get reasonably good styling—GitHub is a little different, but gives a good idea of how it can look: ctrlc. Then screen readers can expose that (or not!) as they choose, but it at least conveys the appropriate semantics. To land it, we will also need to update the existing code which strips out those spans.