Added proposed change to variables shadowing problem 6

solutions/variables.md

@@ -83,8 +83,7 @@ fn main() {
     let mut x: i32 = 1;
     x = 7;
     // Shadowing and re-binding
-    let mut x = x; 
-    x += 3;
+    let x = x;
 
 
     let y = 4;