Minor touchups to variables prog

variables/src/main.rs

@@ -37,7 +37,12 @@ fn main() {
             fib_n(n)
         }
         "rx" => redefine_x(),
-        "help" | _ => show_help(),
+        "help" => show_help(),
+        _ => {
+            show_help();
+            eprintln!("ERROR: missing sub-command");
+            process::exit(1);
+        }
     }
 }
 
@@ -58,7 +63,7 @@ fn redefine_x() {
     println!("The value of x is: {}", x);
 }
 
-fn temp_convert(to_convert: &String) {
+fn temp_convert(to_convert: &str) {
     let to_convert = to_convert.to_lowercase();
     if to_convert.ends_with("c") {
         let c: f32 = to_convert