修正切片练习题2错误

en/src/compound-types/slice.md

@@ -25,9 +25,9 @@ fn main() {
 
     let slice = &arr[..2];
     
-    // Modify '6' to make it work
-    // TIPS: slice( reference ) IS NOT an array, if it is an array, then `assert!` will passed: Each of the two UTF-8 chars '中' and '国'  occupies 3 bytes, 2 * 3 = 6
-    assert!(std::mem::size_of_val(&slice) == 6);
+    // Modify '8' to make it work
+    // TIPS: slice( reference ) IS NOT an array, if it is an array, then `assert!` will passed: Each of the two chars '中' and '国'  occupies 4 bytes, 2 * 4 = 8
+    assert!(std::mem::size_of_val(&slice) == 8);
 
     println!("Success!");
 }