Update Exercises Directory Names to Reflect Order

exercises/21_macros/macros2.rs

@@ -0,0 +1,16 @@
+// macros2.rs
+//
+// Execute `rustlings hint macros2` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    my_macro!();
+}
+
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+}