Add solutions to functions

solutions/01_variables/variables4.rs

@@ -4,6 +4,6 @@ fn main() {
     let mut x = 3;
     println!("Number {x}");
 
-    x = 5; // Don't change this line
+    x = 5;
     println!("Number {x}");
 }

solutions/01_variables/variables5.rs

@@ -1,5 +1,5 @@
 fn main() {
-    let number = "T-H-R-E-E"; // Don't change this line
+    let number = "T-H-R-E-E";
     println!("Spell a number: {}", number);
 
     // Using variable shadowing