manpage: add -u flag in synopsis
manpage: add -u flag in synopsis

Darkanmoore/i3lock_cam,jigkoxsee/i3lock-color,freestyl3r/i3lock,rtfb/i3lock,MartinRogalla/BlueArch,kuruczgyurci/i3lock,justbrowsing/i3lock-spy,Kamori/i3lock,popoffka/i3lock,stettberger/i4lock,koebi/i3lock,kuruczgyurci/i3lock,sebastian-frysztak/i3lock-color,freestyl3r/i3lock,Stautob/i3lock,sm4tik/i3lock,justbrowsing/i3lock-spy,sebastian-frysztak/i3lock-color,ravinrabbid/i3lock-svg,endir/i3lock,popoffka/i3lock,Kha/i3lock,BenElgar/i3lock,ramnes/i3lock,jaseg/i3lock,ravinrabbid/i3lock-svg,kunev/i3lock-color-scale,ravinrabbid/i3lock-clock,eBrnd/i3lock-color,kuruczgyurci/i3lock,shiver/i3lock,romuald/i3lock,tkuipers/i3lock,proxypoke/i3lock