i3lock.1: use signal names without SIG prefix (Thanks rodfersou)
i3lock.1: use signal names without SIG prefix (Thanks rodfersou)

see #32

koebi/i3lock,kuruczgyurci/i3lock,kuruczgyurci/i3lock,sebastian-frysztak/i3lock-color,sebastian-frysztak/i3lock-color,kuruczgyurci/i3lock