Update Exercises Directory Names to Reflect Order

Author: adamhb123

exercises/intro/README.md renamed to exercises/00_intro/README.md

@@ -29,7 +29,7 @@ fn main() {
     println!("or logic error. The central concept behind Rustlings is to fix these errors and");
     println!("solve the exercises. Good luck!");
     println!();
-    println!("The source for this exercise is in `exercises/intro/intro1.rs`. Have a look!");
+    println!("The source for this exercise is in `exercises/intro00/intro1.rs`. Have a look!");
     println!(
         "Going forward, the source of the exercises will always be in the success/failure output."
     );