doc: fix example about shared highlighter group
doc: fix example about shared highlighter group

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