Update Exercises Directory Names to Reflect Order

exercises/02_functions/functions2.rs

@@ -0,0 +1,16 @@
+// functions2.rs
+//
+// Execute `rustlings hint functions2` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    call_me(3);
+}
+
+fn call_me(num:) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}