More sed(1) usage fixing: the extension -i is not optional.

Pointed out by:	jmallet
MFC after:	3 days

usr.bin/sed/main.c

@@ -204,7 +204,7 @@ static void
 usage(void)
 {
 	(void)fprintf(stderr,
-	    "usage: %s script [-Ealnru] [-i[<extension>]] [file ...]\n"
+	    "usage: %s script [-Ealnru] [-i extension] [file ...]\n"
 	    "\t%s [-Ealnu] [-i extension] [-e script] ... [-f script_file]"
 	    " ... [file ...]\n", getprogname(), getprogname());
 	exit(1);