feat: Add variables5.rs exercise

closes #260

exercises/variables/variables5.rs

@@ -0,0 +1,11 @@
+// variables5.rs
+// Make me compile! Execute the command `rustlings hint variables5` if you want a hint :)
+
+// I AM NOT DONE
+
+fn main() {
+    let number = "3";
+    println!("Number {}", number);
+    number = 3;
+    println!("Number {}", number);
+}