Update Exercises Directory Names to Reflect Order

exercises/21_macros/macros4.rs

@@ -0,0 +1,21 @@
+// macros4.rs
+//
+// Execute `rustlings hint macros4` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+#[rustfmt::skip]
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    }
+    ($val:expr) => {
+        println!("Look at this other macro: {}", $val);
+    }
+}
+
+fn main() {
+    my_macro!();
+    my_macro!(7777);
+}