doc.kak: Fix bashism.
doc.kak: Fix bashism.

Fixes #2442.

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