jenkins.sh: fix 'do_build_doc'
jenkins.sh: fix 'do_build_doc'

mfp/lwt,ocsigen/lwt,rneswold/lwt,mirage/lwt,rgrinberg/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,fdopen/lwt,c-cube/lwt,rneswold/lwt,ocsigen/lwt,rgrinberg/lwt,rgrinberg/lwt,mirage/lwt,fdopen/lwt,mfp/lwt,c-cube/lwt,rgrinberg/lwt