Update Exercises Directory Names to Reflect Order

exercises/01_variables/variables6.rs

@@ -0,0 +1,11 @@
+// variables6.rs
+//
+// Execute `rustlings hint variables6` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+const NUMBER = 3;
+fn main() {
+    println!("Number {}", NUMBER);
+}