silence: fix usage string
silence: fix usage string

Don't escape the percent sign in the silence effect usage string, as
it isn't used as a printf format string. Plus a spelling fix.

Motiejus/sox,pcqpcq/sox,Distrotech/sox,Motiejus/sox,davel/sox,cbagwell/sox,MageSlayer/sox,davel/sox,mhartzel/sox_personal_fork,jacksonh/sox,uklauer/sox,Distrotech/sox,cbagwell/sox,mhartzel/sox_personal_fork,uklauer/sox,CaptainHayashi/sox,mhartzel/sox_personal_fork,jacksonh/sox,cbagwell/sox,Distrotech/sox,Distrotech/sox,uklauer/sox,MageSlayer/sox,uklauer/sox,jacksonh/sox,davel/sox,MageSlayer/sox,cbagwell/sox,CaptainHayashi/sox,CaptainHayashi/sox,pcqpcq/sox,Motiejus/sox,pcqpcq/sox,Motiejus/sox,davel/sox,mhartzel/sox_personal_fork,jacksonh/sox,MageSlayer/sox,pcqpcq/sox,CaptainHayashi/sox,MageSlayer/sox