fix 2 key combo marker when substitute() fails

fix laughinghan/dotfiles#2: the 2-key-combo marker characters don't
display on lines with only one non-whitespace character at the very
end (for example, a close-brace `}` ), or nonempty but whitespace-only
lines, because `col_num` is the last column of the line so the regex
fails to match two characters coming after that, so nothing gets
substituted and the 2-key-combo marker characters just don't show up.