added more usage info

src/main.rs

@@ -28,7 +28,7 @@ fn main() {
   let mut arguments: Vec<String> = env::args().collect();
   let argv0 = arguments.remove(0);
   if arguments[1].is_empty() {
-    eprintln!("Usage: {} [table...].[value] [file...]", argv0);
+    eprintln!("Usage: {} [table...].[value[index]] [file...]", argv0);
     process::exit(64); // sysexits(3) EX_USAGE
   }
   let input = &arguments[1];