clarification

src/main.rs

@@ -11,7 +11,7 @@ fn main() {
     let timeout: u64 = args.get(1)
         .expect("There should be at least 1 argument, which specifies amount of time (in seconds) for timer to work!")
         .parse()
-        .expect("The first argument should be a non-negative number");
+        .expect("The first argument should be a non-negative integer number");
     let measurement: char = args
         .get(2)
         .map_or('s', |arg| arg.chars().next().unwrap_or('s'));