fixed usage detection

src/main.rs

@@ -27,7 +27,7 @@ use toml::Value;
 fn main() {
   let mut arguments: Vec<String> = env::args().collect();
   let argv0 = arguments.remove(0);
-  if arguments[1].is_empty() {
+  if arguments.is_empty() {
     eprintln!("Usage: {} [table...].[value[index]] [file...]", argv0);
     exit(64); // sysexits(3) EX_USAGE
   }