ss: replace bfp with bpf in usage().

This patch fixes usage() of misc/ss.c to use bpf instead of bfp.
Signed-off-by: Rami Rosen <ramirose@gmail.com>

misc/ss.c

@@ -2926,7 +2926,7 @@ static void _usage(FILE *dest)
 "   -p, --processes	show process using socket\n"
 "   -i, --info		show internal TCP information\n"
 "   -s, --summary	show socket usage summary\n"
-"   -b, --bfp           show bpf filter socket information\n"
+"   -b, --bpf           show bpf filter socket information\n"
 "\n"
 "   -4, --ipv4          display only IP version 4 sockets\n"
 "   -6, --ipv6          display only IP version 6 sockets\n"