mothra: restore usage fix, document -a option in manual

sys/man/1/mothra

@@ -4,7 +4,7 @@ mothra \- retrieve and display World-Wide Web files
 .SH SYNOPSIS
 .B mothra
 [
-.B -dv
+.B -dva
 ]
 [
 .B -m
@@ -52,10 +52,14 @@ the environment variable
 .PP
 There are a number of options:
 .TP
-.B  -d
+.B -d
 Debug mode. Causes debug messages to appear in
 .B $home/lib/mothra/mothra.err.
 .TP
+.B -a
+Alt display. Starts in alt display mode, see menu
+commands table below.
+.TP
 .B -m
 Specify the
 .IR webfs (4)

sys/src/cmd/mothra/mothra.c

@@ -295,7 +295,7 @@ void main(int argc, char *argv[]){
 	switch(argc){
 	default:
 	Usage:
-		fprint(2, "Usage: %s [-dva] [-m mtpt] [url]\n", argv[0]);
+		fprint(2, "Usage: %s [-dva] [-m mtpt] [url]\n", argv0);
 		exits("usage");
 	case 0:
 		url=getenv("url");