docs: clarify variables5 mutability

exercises/01_variables/README.md

@@ -3,6 +3,7 @@
 In Rust, variables are immutable by default.
 When a variable is immutable, once a value is bound to a name, you can't change that value.
 You can make them mutable by adding `mut` in front of the variable name.
+`variables5` is about that `mut` keyword, not shadowing.
 
 ## Further information
 

exercises/01_variables/variables5.rs

@@ -2,7 +2,8 @@ fn main() {
     let number = "T-H-R-E-E"; // Don't change this line
     println!("Spell a number: {number}");
 
-    // TODO: Fix the compiler error by changing the line below without renaming the variable.
+    // TODO: Fix the compiler error by making `number` mutable.
+    // This exercise is about mutability, not shadowing.
     number = 3;
     println!("Number plus two is: {}", number + 2);
 }