swab(1): use the getopt error message

src/swab.rs

@@ -52,7 +52,7 @@ fn main() -> ExitCode {
 	let mut optind: usize = 1;   // argv[0]
 	let mut wordsize: usize = 2; // Equivalent to dd(1p).
 
-	while let Some(opt) = argv.getopt(":w:") {
+	while let Some(opt) = argv.getopt("w:") {
 		match opt.opt() {
 			Ok("w") => {
 					match opt.arg().unwrap().parse::<usize>() {