zig.kak: fix highlighting of empty doc comment
zig.kak: fix highlighting of empty doc comment

If a line contains three slashes directly followed by a new line, the
next line is also erroneously highlighted as a doc comment currently.
Using a lookahead instead fixes this.

Somasis/kakoune,casimir/kakoune,lenormf/kakoune,mawww/kakoune,danr/kakoune,danr/kakoune,Somasis/kakoune,mawww/kakoune,danr/kakoune,casimir/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,Somasis/kakoune,alexherbo2/kakoune,danr/kakoune,lenormf/kakoune,casimir/kakoune,lenormf/kakoune,casimir/kakoune,mawww/kakoune,jkonecny12/kakoune,Somasis/kakoune,alexherbo2/kakoune,mawww/kakoune,alexherbo2/kakoune,lenormf/kakoune,jkonecny12/kakoune