README: clarify that change commands work on each selection
README: clarify that change commands work on each selection

I think "current selection" could be misinterpreted as "main
selection", so let's use "each selection".

jkonecny12/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune