Update Exercises Directory Names to Reflect Order

exercises/01_variables/variables3.rs

@@ -0,0 +1,11 @@
+// variables3.rs
+//
+// Execute `rustlings hint variables3` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    let x: i32;
+    println!("Number {}", x);
+}